>I think of Math Academy as ideal for efficiently mastering subjects at a level slightly deeper than a typical education in a shorter amount of time, and AoPS is for taking your time to go much deeper into the topics.

I've been searching hi and lo for the ultimate beginner resource and nothing comes close to AoPS for the exact reason you mentioned. I started with principia mathematica by bertrand russel because I thought math worked like a big tree all stemming from one trunk. Boy was that a bad move lol. I'd be reading some textbook and I have an infinite amount of why's that would block me on the most basic of things, tons of assumed knowledge. AoPS treats you like a caveman who first discovered fire, and then discovered their books, in that order.

I'll give MathAcademy a go too, I heard lots of great things.