I made some things with the data modify command (Enchantment infinity, which allow you to gather 2 same enchants together in order to get the higher one, can be done endless. Also move enchantments, and some amazing features)
But as I saw, if there's nothing in a slot, the list won't take it. As a result, you can't get a slot by Inventory.
the /data mode can't search for slots, they only search in the set path...
Searching as Inventory.slot.103 wont work, as the command is expecting for a list after "Inventory"
I can't test it right now, minecraft can't launch :/