You can do that manually. Say for example you learn integration by trig sub today and you do 30 problems from a book. Next week you do some more trig sub problems. Then 2-3 weeks after that you do some trig sub problems and then in a few months you do some. You can do spaced rep manually. Is mathacademy more efficient? I don't know. It's too early to say. But what I do know is millions of people have learned mathematics with books, pen and paper for hundreds of years.

> You can do that manually.

Absolutely. You can spend time on figuring out what to do next, and how, and how to do spaced repetition for the material and test yourself effectively. There are aspects you'll do better than a set curriculum because you understand yourself, and there are mistakes you'll make because misunderstandings and errors.

Or you can pay an expert to do that for you, and just use the time on learning.