I guess I am of the few that would pay for a good tool aimed at serious learners, but as I could not find any (in language space almost everything is a Duolingo clone), I am building myself.

So far I think I have proved my main hypothesis: it works really well for me. It's not magic, but it saves me many hours of learning.

Whether it could work for someone else, is an entirely different story... but on the other hand, I have no big ambitions.