Many people complain that there should not be a level-limit to anvils. However, I have never encountered this issue IF I’m using it properly. I have made the perfect armor / tools with all the best enchants multiple times (on vanilla), and if you combine the books in the correct order, you never reach the limit.
Mending is not always available in those stages, especially for loner saves (no/minimal village usage). Until now you have to farm librarians to have it.