If you think of something like Anki for drilling problems/formulas, it's like that but built for math and more convenient: no creating your own cards, it works well on mobile and helps track your progress over time.
It uses FSRS for scheduling and includes over 40,000 problems ranging from 1st grade math through university level math, all deterministically verified. It also has levels and dozens of badges you can earn to keep it engaging.
You can use it via the iOS app or the web app. The iOS app is free and completely offline aside from optionally sending feedback/requesting help in app. In the web version progress is stored locally in IndexedDB, except in challenge mode where you can optionally submit initials to the leaderboard.
I still use Math Academy to learn math (highly recommended), but as I'm learning more advanced math to support my journey into AI and robotics I wanted to try and raise my own ceiling by building automaticity in stuff I already learned.
The idea is a little counterintuitive: if Mathy works, you should eventually need it less. Because as something becomes automatic, its recommended reviews can move farther apart thanks to FSRS.
Edit: More about how I made it here https://gmays.com/making-math-automatic-with-mathy/