Honestly the best bet is to just read the best books for a given subject. You can figure out which books are used in curriculums by checking syllabi. Seems reasonable that if they are using the books at Harvard/Yale/MIT/Caltech/Stanford that they are probably decent.

I personally like to read a few books on the same subject in a row. It might be helpful to use wolfram alpha, SAGE, or chatgpt to explain concepts or help answer questions. Also buy a solutions manual and use it to check your answers.

As someone who did a math degree at a decent institution, I can assure you learning on your own (assuming you have the discipline) is a lot less stress than taking a class and you’ll likely learn more.

If you’re really motivated, hire a TA in the subject you are studying (online meets are fine). If you need to, just walk into the math department at a local university and ask where you can find tutors for a specific class. Just tell them you’re planning to enroll but you can’t right now.

Or check out a multitude of mathematics communities on discord or slack and ask around.