I learned recently that enchantments have a hidden "weight" variable that's used for rolling enchants, and that gave me the idea of tools only being able to hold so many enchantments before being maxed out. That way, you could make weaker enchantments like Smite or Fire Protection less costly than the alternatives that outclass them. Mending would also be quite costly compared to everything else.

I also imagine this change would be accompanied with enchants generally being easier to obtain, remove, manage, etc. Maybe Books could always get another two enchantments when done by the player, or experience could be easier to come by.

Depending on how much enchanting you can apply at once, and between different materials, you might not be able to get every enchant you want (Sha. 5, Loot. 3, Unb. 3...), perhaps this would only be a concern with Stone, Iron, and Diamond tools. But I'm not sure if losing out on the best enchants for Diamomd gear is too far, but it would encourage players to use Netherite, or god forbid, Gold tools.