As a follow up: I want to pursue a math degree study. What course titles and textbooks starting at the calculus level do you guys recommend? I want enough math chomps to then go onto a PhD in ML.
Linear Algebra (which is what you really need): Gilbert Strang - Linear Algebra and its applications
These two are all you need, with which you'll get a solid base. Then you're good to go on your own. These two combined are about 4 semesters worth of work. But if you really focus, I think you can get them done in a little less than 6 months.
If you want a 'just what I need' approach, Khan Academy.
I don't remember what intro linear algebra books I used, but my college uses this: https://www.math.ucdavis.edu/~linear/ (I took the class before this free textbook was developed).
https://np.reddit.com/r/MachineLearning/comments/73n9pm/d_co...