Math Academy uses spaced repetition for skills with tiny, to-the-point interactive lessons (typically following "theory, some exercises, theory, some more exercises" formula) based on an initial diagnostic test, where the skills are structured as a graph of dependencies.

I didn’t, at the time, appreciate how challenging a problem it was until I started researching Bayesian Knowledge Tracing. While their definition of a skill can be a bit narrow, thus putting more time into reviewing things I'd rather move on from, it does work from what I've observed.

I recall they had a course on Abstract Algebra and other more advanced subjects, so if you're really interested, the great thing about subscriptions is that you can afford to try it.