I know this kind of datapack already exist, but here is another way to do it if you don't like the others.

You don't need to destroy your anvil and put it in a crafting table or in a smithing table, just drop a iron ingot into to repair your anvil.
Just a little tip if you ask, there is a little probability to destroy your anvil when using it, so there is a probability to repair it.

PS: Tell me if there is some bug and I will correct them, I can buff or nerf the repair if you ask :3

Made in 1.16.3