What you're looking for is not something to help your with your goals, it's something that sells you the idea that you can achieve your goals while seemingly be unwilling to put the necessary effort in.

What if it's simply impossible to learn linear algebra and mid-level math stuff by spending only 30 minutes a day? What if you can't get fit if you don't go to the gym?

It sounds like what they are looking for is a pathway. Khan used to have a really good one with the world of math pathway where you could go for 2 + 2 to Calculus in a set sequence of steps each building on the next. I've tried Khan a few times since they got rid of it and it just feels like you're all over the place and regularly missing skills it expects you to have.

They aren't trying to get fit by not going to the gym they are looking for a progressive overload program for math.