Spawn Pets

I took at look at your datapack, and considering what it does, its not performant or functional enough for me to consider linking it. A 10 command run per tick is not the best way to handle this. Its also only targeting the selected item meaning that if the player doesn't hold it in their mainhand, it would never work.

You need to add all the items into a item tag. On inventory update, call an advancement that reward triggers a scan function of the whole player inventory and if the tagged item is found, update it as equippable. That way, it does a 51 command run per update which is far more performant when considering a 10 command per tick (using tag makes it a single command). It also solves the selected item issue so the player can have it anywhere in their inventory for it to update. All this works without external mods.

Datapacks can tho. I have multiple packs that modifies the player inventory.