Crafting: Detecting NBT (or alternate) Data [1.21.1]

I just got into data pack creation and have run into a lot of issues. Thankfully, I've been able to solve them all, but this one stumped me. I'm attempting to add pebbles into minecraft--as a sort of easy, starter project (or so I think). The base item I'm using is minecraft:iron_nugget, and as such, players can craft iron ingots out of pebbles! To prevent this, I added custom item data to the item:
                  "function": "minecraft:set_custom_data",
                  "tag": "{isPebble:1b}"
I wanted to change the base recipe to exclude iron nuggets with this tag (as to prevent players from using pebbles to craft iron ingots):
                 ...
                   "key": {
                      "#": {
                        "item": "minecraft:iron_nugget",
                        "nbt": "{isPebble:0b}"
                      }
                    }
However, I can't seem to get it to work--nor do I exactly know how to go about doing this.
If someone could point me in the right direction, that would be great!!
-------------------------------------------------------Files (if needed)-------------------------------------------------------
Iron ingot recipe code (iron_ingot_from_nuggets.json):
https://hastebin.skyra.pw/irukoxuher.css
I made pebbles drop from short grass, so the code for the iron nugget/pebble is in the short grasses loot table (short_grass.json):
https://hastebin.skyra.pw/uyofexagex.prolog