Your own learning resources + Anki.

However, this requires significantly larger amount of work compared to said learning apps, which has establisged learning course in form of easily digestable chunk.

You need to digest textbook and such on your own into small chunks, then Anki cards. It’s essentially building course by yourself.

Fluent Forever from Gabriel Wymer gives some idea on how to utilize Anki, though the author moved to build his own closed source app.