i think that it is key to play scales, first just at 80 bpm, 2 beats per note, using arm and finger and a slow landing for each note, focusing on tone, then doing 1 per beat, 2 octs 2 per beat, 3 octs 3 per beat, 4 per beat contrary etc etc 6 and (if you can) 8 …
to do major and minor arpeggios in the scale in all of its inversions, first slowly with metranome, then more quickly (measuring with the beat in 3 per beat, 4, then 6, whatever) and then dominant chords in all of the inversions, always starting on the thumb and pinky even if it’s a black key, finishing with the diminished arpeggio… ALSO, broken chords in the same manner.
I do this for every piece I do, whatever key it is in… so if it’s d minor, i do all of this for d minor. It only takes 15-20 minutes tops, and the arpeggios (especially dominant) give your fingers such dexterity, and teaches them to stretch all shapes, not to mention you’ll memorize more quickly and recognize all of these (SUPER HIGHLY) common shapes in the music. Doing a Harmonic scale in octaves is also a descent idea.
Other then this, i’d say keep it up with WTC, even do a Suite or Partita (if you can memorize quickly, if not stick to WTC pieces), Bach is always excellent for dexterity, for phrasing, for memorizing, for the ear… and to work on Chopin etudes, even if you can’t get them up to speed, one at a time.
If you get to a certain point where the scales and arpeggios get too easy, I’d recommend using the Liszt excersizes, because that’s taking the arps and scales and giving them CRACK
this is just what I think is ideal and what i’d teach my students if I had any advanced ones (i teach for some money during summers)
When you are bored of memorizing music, or working on something, try the Hannon trills excersize (i actually think its a good one) or something else. It’s a good break (mindless practice does suck, but I really don’t think you need much focus to do this excersize)
Now, what I’ve found helps you improve more than anything, is to make sure your relaxed no matter what you are playing (even octaves), making sure that your arms are guiding you for the thumbs crossing over in the arpeggios, that you’re not twisting your hand too much, shoulders are low, etc etc
then you truly master the piano