My question is what technique should I focus on. In bar 20, I am using the tremolo, but in the passage of the chord there are 2 doubts: do I need to keep finger 1 pressed D # while I bring the other fingers to the next chord or should I move it up first to make the passage later? The other question, is in bars 21 and 22, I must shake between the fingers of the extremities (finger 1/5 in bar 21, finger 1/4 in bar 22) while the central finger just strums, or I must keep my wrist without oscillation (or with very little oscillation) while just typing as keys?
Starting in 21 with that fast Alberti bass, most if not all teachers will recommend rotating your wrist to alternate between finger 1 and the lower fingers. Even the finger playing the middle note should be supported by wrist rotation. Josh Wright very recently posted an audio-only lesson on this on YouTube:
For the alternating bass 8th notes in most of measures 1-20, I would have a little wrist rotation combined with a slight left right motion of the arm so you don’t have to stretch the fingers too much.
With a piece like this you really should find a competent teacher even if you only have a lesson once in a while. Not all teachers require a weekly commitment.
doesn't 5 and 2 make more sense for the first bar (and similar previous bars)? and 4121 for the second? but i'm not convinced there should be focus on wrist rotation. i think what i mean by this is: these LH passages are technically straightforward relative to the rest of the piece, so if they're offering difficulty then it questions the appropriateness of learning that piece.
i agree with the above comment about finding a competent teacher for a piece like this.