Completely redone the stack modifying:

It now runs only for all items you want to change (instead of all registered items)
You can now use tags to modify the stack size of all items with that tag, includes convention tags (#c:<tag_name>)
You no longer need to restart the game after making a change to the config, simply pressing the done button will make changes take effect

src/main/java/github/mrgii/itemsstack/ItemsStack.java

@@ -4,15 +4,23 @@
 import github.mrgii.itemsstack.util.ItemsStackSizeModifier;
 import net.fabricmc.api.ModInitializer;
 
+import net.fabricmc.fabric.api.event.lifecycle.v1.ServerLifecycleEvents;
+import net.fabricmc.fabric.api.tag.convention.v2.ConventionalItemTags;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 public class ItemsStack implements ModInitializer {
-    public static final Logger LOGGER = LoggerFactory.getLogger("potionsstack");
+    public static final Logger LOGGER = LoggerFactory.getLogger("itemsstack");
 	public static final ItemsStackSizeConfig CONFIG =  ItemsStackSizeConfig.createAndLoad();
 
 	@Override
 	public void onInitialize() {
-		ItemsStackSizeModifier.ModifyItemStackSize(CONFIG.items(), CONFIG.maxStackSizes());
+		String tagName = ConventionalItemTags.TOOLS.getName().getString();
+		LOGGER.info(tagName);
+		ServerLifecycleEvents.SERVER_STARTED.register((minecraftServer) -> {
+			ItemsStackSizeModifier.updateValuesAndModifyStackSizes();
+			ItemsStackSizeModifier.registerCallbacks();
+		});
+
 	}
 }

src/main/java/github/mrgii/itemsstack/ItemsStackClient.java

@@ -1,9 +1,18 @@
 package github.mrgii.itemsstack;
 
+import github.mrgii.itemsstack.util.ItemsStackSizeModifier;
 import net.fabricmc.api.ClientModInitializer;
+import net.fabricmc.fabric.api.client.event.lifecycle.v1.ClientLifecycleEvents;
+import net.fabricmc.fabric.api.client.networking.v1.ClientPlayConnectionEvents;
 
 public class ItemsStackClient implements ClientModInitializer {
     @Override
     public void onInitializeClient() {
+        ClientPlayConnectionEvents.JOIN.register((clientPlayNetworkHandler, packetSender, client) -> {
+            ItemsStackSizeModifier.updateValuesAndModifyStackSizes();
+        });
+        ClientLifecycleEvents.CLIENT_STARTED.register((client) -> {
+            ItemsStackSizeModifier.registerCallbacks();
+        });
     }
 }

src/main/java/github/mrgii/itemsstack/config/ItemsStackSizeConfigModel.java

@@ -8,11 +8,15 @@
 @Modmenu(modId = "itemsstack")
 @Config(name = "items-stack-size-config", wrapperName = "ItemsStackSizeConfig")
 public class ItemsStackSizeConfigModel {
-    @RestartRequired
     @Sync(SyncMode.OVERRIDE_CLIENT)
     public List<String> items = List.of();
 
-    @RestartRequired
     @Sync(SyncMode.OVERRIDE_CLIENT)
     public List<Integer> maxStackSizes = List.of();
+
+    @Sync(SyncMode.OVERRIDE_CLIENT)
+    public List<String> itemTags = List.of();
+
+    @Sync(SyncMode.OVERRIDE_CLIENT)
+    public List<Integer> maxTagStackSizes = List.of();
 }

src/main/java/github/mrgii/itemsstack/mixin/ItemEntityMixin.java

@@ -2,7 +2,6 @@
 
 import com.llamalad7.mixinextras.injector.ModifyExpressionValue;
 import net.minecraft.entity.ItemEntity;
-import net.minecraft.item.ItemStack;
 import org.spongepowered.asm.mixin.Mixin;
 import org.spongepowered.asm.mixin.injection.At;
 

src/main/java/github/mrgii/itemsstack/util/ItemsStackSizeModifier.java

@@ -1,32 +1,110 @@
 package github.mrgii.itemsstack.util;
 
 import github.mrgii.itemsstack.ItemsStack;
+import github.mrgii.itemsstack.config.ItemsStackSizeConfig.Keys;
+import io.wispforest.owo.config.Option;
+import net.fabricmc.fabric.api.tag.convention.v2.ConventionalItemTags;
 import net.fabricmc.fabric.mixin.item.ItemAccessor;
 import net.minecraft.component.ComponentMap;
 import net.minecraft.component.DataComponentTypes;
 import net.minecraft.item.Item;
 import net.minecraft.registry.Registries;
+import net.minecraft.registry.tag.ItemTags;
+import net.minecraft.registry.tag.TagKey;
+import net.minecraft.util.Identifier;
 
+import java.lang.reflect.Field;
 import java.util.List;
 import java.util.Map;
 
 public class ItemsStackSizeModifier {
-    public static void ModifyItemStackSize(List<String> itemNames, List<Integer> maxStackSizes) {
-        Map<String, Integer> itemToMaxStackSize = createItemToStackSizeMap(itemNames, maxStackSizes);
-        ItemsStack.LOGGER.info("Changing stack size of minecraft potions.");
-        for (Item item : Registries.ITEM) {
-            String name = item.getName().getString();
-            if (itemToMaxStackSize.containsKey(name)) {
-                ((ItemAccessor) item).setComponents(ComponentMap.of(item.getComponents(), ComponentMap.builder().add(DataComponentTypes.MAX_STACK_SIZE, itemToMaxStackSize.get(name)).build()));
-            }
+    public static List<String> itemNames;
+    public static List<Integer> maxStackSizes;
+    public static List<String> itemTagNames;
+    public static List<Integer> maxTagStackSizes;
+
+    public static void ModifyItemStackSizes() {
+        Map<Item, Integer> itemToMaxStackSize = createItemToStackSizeMap(itemNames, maxStackSizes);
+        Map<TagKey<Item>, Integer> itemTagToMaxStackSize = createItemTagToStackSizeMap(itemTagNames, maxTagStackSizes);
+        ItemsStack.LOGGER.info("Changing stack size of minecraft item tags.");
+        for (TagKey<Item> tagKey : itemTagToMaxStackSize.keySet()) {
+            Registries.ITEM.iterateEntries(tagKey).forEach((itemRegistryEntry) -> {
+                Item item = itemRegistryEntry.value();
+                ((ItemAccessor) item).setComponents(ComponentMap.of(item.getComponents(), ComponentMap.builder().add(DataComponentTypes.MAX_STACK_SIZE, itemTagToMaxStackSize.get(tagKey)).build()));
+            });
+        }
+        ItemsStack.LOGGER.info("Changing stack size of minecraft items.");
+        for (Item item : itemToMaxStackSize.keySet()) {
+            ((ItemAccessor) item).setComponents(ComponentMap.of(item.getComponents(), ComponentMap.builder().add(DataComponentTypes.MAX_STACK_SIZE, itemToMaxStackSize.get(item)).build()));
         }
     }
 
-    private static Map<String, Integer> createItemToStackSizeMap(List<String> itemNames, List<Integer> maxStackSizes) {
-        Map<String, Integer> itemToMaxStackSize = new java.util.HashMap<>(Map.of());
+    private static Map<Item, Integer> createItemToStackSizeMap(List<String> itemNames, List<Integer> maxStackSizes) {
+        Map<Item, Integer> itemToMaxStackSize = new java.util.HashMap<>(Map.of());
         for (int i = 0; i < Math.min(itemNames.size(), maxStackSizes.size()); i++) {
-            itemToMaxStackSize.put(itemNames.get(i), maxStackSizes.get(i));
+            itemToMaxStackSize.put(Registries.ITEM.get(Identifier.of(itemNames.get(i))), maxStackSizes.get(i));
         }
         return itemToMaxStackSize;
     }
+
+    private static Map<TagKey<Item>, Integer> createItemTagToStackSizeMap(List<String> itemTagNames, List<Integer> maxStackSizes) {
+        Map<String, Integer> itemTagToMaxStackSize = new java.util.HashMap<>(Map.of());
+        for (int i = 0; i < Math.min(itemTagNames.size(), maxStackSizes.size()); i++) {
+            itemTagToMaxStackSize.put(itemTagNames.get(i), maxStackSizes.get(i));
+        }
+        Map<TagKey<Item>, Integer> itemTagKeyToMaxStackSize = new java.util.HashMap<>(Map.of());
+        Field[] conTagKeyFields = ConventionalItemTags.class.getDeclaredFields();
+        for (Field conTagKeyField : conTagKeyFields) {
+            conTagKeyField.setAccessible(true);
+            TagKey<Item> conTagKey;
+            try {
+                conTagKey = (TagKey<Item>) conTagKeyField.get(null);
+            } catch (IllegalAccessException e) {
+                ItemsStack.LOGGER.error("Could not access tagKeyField {}!", conTagKeyField.getName());
+                throw new RuntimeException(e);
+            }
+            String conTagName = "#" + conTagKey.id().toString();
+            if (itemTagToMaxStackSize.containsKey(conTagName)) {
+                itemTagKeyToMaxStackSize.put(conTagKey, itemTagToMaxStackSize.get(conTagName));
+            }
+        }
+        Field[] tagKeyFields = ItemTags.class.getDeclaredFields();
+        for (Field tagKeyField : tagKeyFields) {
+            tagKeyField.setAccessible(true);
+            TagKey<Item> tagKey;
+            try {
+                tagKey = (TagKey<Item>) tagKeyField.get(null);
+            } catch (IllegalAccessException e) {
+                ItemsStack.LOGGER.error("Could not access tagKeyField {}!", tagKeyField.getName());
+                throw new RuntimeException(e);
+            }
+            String tagName = tagKey.getName().getString();
+            if (itemTagToMaxStackSize.containsKey(tagName)) {
+                itemTagKeyToMaxStackSize.put(tagKey, itemTagToMaxStackSize.get(tagName));
+            }
+        }
+        return itemTagKeyToMaxStackSize;
+    }
+
+    public static void updateValuesAndModifyStackSizes() {
+        ItemsStackSizeModifier.itemNames = ItemsStack.CONFIG.items();
+        ItemsStackSizeModifier.maxStackSizes = ItemsStack.CONFIG.maxStackSizes();
+        ItemsStackSizeModifier.itemTagNames = ItemsStack.CONFIG.itemTags();
+        ItemsStackSizeModifier.maxTagStackSizes = ItemsStack.CONFIG.maxTagStackSizes();
+
+        ModifyItemStackSizes();
+    }
+
+    public static void registerCallbacks() {
+        Keys keys = ItemsStack.CONFIG.keys;
+        Option<List<String>> itemsOption = ItemsStack.CONFIG.optionForKey(keys.items);
+        Option<List<String>> maxStackSizesOption = ItemsStack.CONFIG.optionForKey(keys.maxStackSizes);
+        Option<List<String>> itemTagsOption = ItemsStack.CONFIG.optionForKey(keys.itemTags);
+        Option<List<String>> maxTagStackSizesOption = ItemsStack.CONFIG.optionForKey(keys.maxTagStackSizes);
+
+        itemsOption.observe(newValue -> ItemsStackSizeModifier.updateValuesAndModifyStackSizes());
+        maxStackSizesOption.observe(newValue -> ItemsStackSizeModifier.updateValuesAndModifyStackSizes());
+        itemTagsOption.observe(newValue -> ItemsStackSizeModifier.updateValuesAndModifyStackSizes());
+        maxTagStackSizesOption.observe(newValue -> ItemsStackSizeModifier.updateValuesAndModifyStackSizes());
+    }
 }

src/main/resources/assets/itemsstack/lang/en_us.json

@@ -2,8 +2,12 @@
   "text.config.items-stack-size-config.title": "Items Stack Config",
 
   "text.config.items-stack-size-config.option.items": "Items",
-  "text.config.items-stack-size-config.option.items.tooltip": "The Names of Items to change the Max Stack Size of.\nThis is the Item's Id, without the 'minecraft:' (or '<modname>:' for mod items),\nevery _ is replaced with a space and the first letter of each word is capitalized.\nEach Item needs to have a matching entry in Max Stack Sizes.",
-
+  "text.config.items-stack-size-config.option.items.tooltip": "The ids of Items to change the Max Stack Size of.\nEach Item needs to have a matching entry in Max Stack Sizes.",
   "text.config.items-stack-size-config.option.maxStackSizes": "Max Stack Sizes",
-  "text.config.items-stack-size-config.option.maxStackSizes.tooltip": "The Max Stack Sizes of each Item in Items.\nThis should be a positive integer.\nThe Max Stack Size is applied to the Item in the same place in Items."
+  "text.config.items-stack-size-config.option.maxStackSizes.tooltip": "The Max Stack Sizes of each Item in Items.\nThis should be a positive integer.\nThe Max Stack Size is applied to the Item at the same place in Items.",
+
+  "text.config.items-stack-size-config.option.itemTags": "Item Tags",
+  "text.config.items-stack-size-config.option.itemTags.tooltip": "The ids of Item Tags to change the Max Stack Size of.\nEach Item Tag needs to have a matching entry in Max Tag Stack Sizes.\nItems that are both part of an Item Tag in this list and are in the Items list\nwill get the value assigned to them from the Items list",
+  "text.config.items-stack-size-config.option.maxTagStackSizes": "Max Tag Stack Sizes",
+  "text.config.items-stack-size-config.option.maxTagStackSizes.tooltip": "The Max Stack Sizes of each Item Tag in Item Tags.\nThis should be a positive integer.\nThe Max Stack Size is applied to the Item Tag at the same place in Item Tags."
 }