As you mentioned, I don’t think it gets much better than khan academy. It’s an incredible source of education and you can refresh on stuff like algebra there.

Alternatively you could watch free lectures from schools online. MIT has open lecture videos for any subject https://ocw.mit.edu/search/

A lot of people would just upvote the first bozo to reply with a confident answer and never bother to check for the full story. Scrolling 3 comments down you’d find someone who actually knows their stuff. I learned to at least read most of the comments before committing any Reddit info to memory as fact.