There is no distinction between stacks and single items. If you throw a stack in, the whole stack gets washed

I would either have to replace all wool blocks with usebale items (like a fishing root, consumables) or spawn an invis villager in the cauldron. I don't realy want to spawn any extra entities or replace blocks, as this datapack should stay lightweight and as feel as vanilla as possible.

Thats already a vanilla feature :). Try clicking on a cauldron with the leather armor in your hand

Maybe in Bedrock edition, but not in Java edition