[dhkxh]

> I’ve found that even trying to really study through books things like number theory, proofs, and integration takes huuuuge time investments.

Yep, that's how to do it if you want to do it well. Practice. There's a reason why these degrees take so long. I would advise against just about any online course where you only watch videos and get the feeling that you've learned something while in reality you retain close to nothing.