How can I keep my little "repertoire" without forgetting how to play the pieces
in that little time and how can I continue to learn new pieces?
What would be the most constructive way to go about this?
If sufficiently learnt, your pieces will hold surprisingly well in your mind even if you don't play them for a reasonable amount of time (I find things are quite strong despite a 5+ year hiatus) And if (they will) do get "lost" you will be able to re-learn them EXTREMELY quickly comparitively to the first time around..
This learn, forget, re-learn process will also cement them even further in your mind.
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Scales are certainly of value to you, - the question is rather what kind of value.. and if you are expecting to master you scales technically and that will in turn make you a good pianist you're chasing something that will ultimately waste a lot of your time.. It is but one piece of the pie.
Familiarity with scale like patterns (similar motion scales are just one of these) in all keys will help you with having a feel for the configurations of notes that generally appear in said keys.. but, in the literature, while there are "scale passages" there is rarely an exact standard scale with the exact standard fingering.. so in reality, while you need to understand and know the notes in a scale, playing the scale will not solve the technical problems in your pieces.. the real solution arrives when you have played countless variants of scale notes/patterns with numerous fingerings.. this is best found simply playing pieces. Which provides the added advantage of being "musical" - by which I mean there is room for dynamic variation in the notes with a genuine emotional intent.. which is a VERY big piece of the "good pianist pie"
You can find some very scalar passages in clementi's sonatinas as a start. - though I have Zero idea whether you are ready for that or beyond that based on your post.
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The real value of scales is in that they are the SIMPLEST and EASIEST way to familiarise yourself with ALL the available tonal options within a KEY... That is that a firm understanding of these "collections" of notes will improve your reading, understanding of harmony, improvisation/composition.. however that requires are more in depth approach to their study than simply playing them them in similar motion. It is about understanding them not being able to play them.