Where can I do a self-study math degree online?

Y Combinator:

My background is in CS and I’m currently working as an ML engineer, but I’m very eager to go back for a PhD soon.
In the mean time, I’ve noticed my maths isn’t as strong as I’d like it to be. That aside, I’ve also taken a big interest into calculus.

Are there any online courses (free or non-free) that would allow me to learn mathematics either through self study, or as a MOOC? I’ll gladly take undegrad level courses. I just want something structured.

I’ve found that even trying to really study through books things like number theory, proofs, and integration takes huuuuge time investments. I feel like this will be less so with a structured course and teacher.