You mention Khan Academy in your post and was going to suggest that - on similar threads that seemed like a highly “rated” recommendation. It’s less of a defined web course/series but seems pretty good for picking and choosing different topics to get into, there’s a lot of coherence with everything IMO. Maybe udemy could be good if you like web courses with linearity - not sure what they have for the math you’re looking to do but at least try to get courses when they’re on sale. Good luck