It depends what the OP call core math competency. If you mean the ability to solve and think critically about problems, I like a combination of "How to solve i

It depends what the OP call core math competency. If you mean the ability to solve and think critically about problems, I like a combination of "How to solve it" by Pólya (https://en.wikipedia.org/wiki/How_to_Solve_It https://en.wikipedia.org/wiki/How_to_Solve_It) for more general ideas and regularly finding puzzles onm different topics such as the ones proposed here (https://fivethirtyeight.com/tag/the-riddler/ https://fivethirtyeight.com/tag/the-riddler/). For deeper mathematics, I feel like that's hard. I come back to my notes from Grad School once in a while, or try to follow proofs for topics of interests. But to be honest, most of them are a little beyond me at times. Maybe Fermat's library (https://fermatslibrary.com/ https://fermatslibrary.com/) can offer some annotated reading which would be helpful? Regardless, I wish you good luck. Please do share if you have any more good ideas.