I've tried other mods that uncap anvil levels and they don't play nice with PiTweaks. I'd love a way to do so from within PiTweaks, which will really make the enchantments be about how much time and effort you want to put into adding levels to your tools as opposed to having to stop when you hit 40 levels.

After trying Unbreaking 21 (which costs 42 levels, by calculation) it works, but the anvil still says "Too expensive!"

This appears to be undocumented behavior. Can you make it clear this isn't "too expensive" by showing what the actual cost will be in green text?

I keep looking at the source to see if there's any way to accomplish it myself, but sadly it looks like there's simply an event in vanilla handling that which is simply not touched by tweak code here. I'd have just as hard a time figuring out what to do in order to add it as anyone else. In addition, even if I did I'd target 1.12.2 and would have no idea how to update it for later versions.