The Art of Problem Solving series of books is pretty much exactly what was described, is relatively affordable to an adult with disposable income, and they all

The Art of Problem Solving series of books is pretty much exactly what was described, is relatively affordable to an adult with disposable income, and they all come with complete solution manuals: https://artofproblemsolving.com/store/list/aops-curriculum https://artofproblemsolving.com/store/list/aops-curriculum You could spend a year or two doing ~20 problems per day while scheduling review of definitions you've understood and problems you've solved with spaced-repetition software like Anki. Afterwards, you'd be prepared for any undergraduate mathematics curriculum in the world.