I don't really see the issue you have here
Minecraft shouldn't change for a small group of minecraft servers
and there is a renewable way to get mending, villagers
you can easily just reroll trades on a librarian till you get mending for a cheap price
and gamerule commands don't disable advancements lmao, and even then there doesn't need to be one for mending
its a hard to acquire enchantment so it warrants being this powerful
