Adding warm-ups by level alone would improve the app. I find myself struggling to warmup on my own because my bias takes me to certain workouts or songs and the scales/arpeggios are too specific (not enough variability) at least up to level 5.

I agree. Whenever I do a guided lesson it offers the songs I need to gold-star regardless of their level (and it might be a 3rd level song that I have once played for a weekly challenge or something, while I'm currently struggling with let's say a Bach bassline from level 7).
If I skip a couple songs suggested for the warm-up it just skips the warm-up altogether and marks it as "done", but I haven't warmed up at all. So there is definitely space for improvement there.

There is a problem for sure. For both guitar and piano, at level five or six, the warm ups are still level 2 or 3, absurdly easy relative to what I am otherwise working on. This makes the "Lesson" modules feel useless and a waste of time. This problem is worse when the more difficult levels take a longer time to get through. Why not add scales or some of the many other exercises or songs? Even add new songs, form lower levels, that the user has never played? Why play "Twinkle, Twinkle" at every single warm up?

I am not sure what the above suggestion is, but I do agree there is something odd about the warm-ups that are provided. Often way below current level. It may be a product of the major upgrade. But it should be looked into.