As an amateur guitarist, I have to time-budget my daily practice. Thus, I can either learn new material or play through the compositions I want to keep in good condition for performance. Playing through ten compositions exactly once already takes up to an hour, which is considerable for me. So I'm wondering if there are solutions to this - maybe play each composition every X days? Or don't repeat them regularly, but only revive them a few weeks before a performance?
There are stories about Glenn Gould 'practicing mentally' rather than physically at the piano. Of course he had extraordinary talent and memory, and mere mortals can't do exactly as he did. But the story suggests that repeated play through might not be the only way to keep a piece of music in memory. I think some people talk about 'visualizing' performance mentally.
Another thing I have tried to aid memorization is to practice a piece by transposing it to another key. This obviously takes us beyond the fingering. It become more like a challenge of 'do you really know what this music is doing?' The hope is better memory retention, because the focus shifts to the pure music instead of fingerings.
I can't make a strong claim either of these ideas will help, because I think my musical memory isn't good. But I hope it offers you some help.
Not having enough time to spend with an instrument will always be challenging to improve your skills.
However, as per answer above, practising away from the instrument can be done:
I always practise! In the shower, walking the dog, driving, waiting for the kettle.... you get the idea.
It is hard to start and takes discipline but with perseverance, it will work.
At one point, I got so much stuff to practise that it took over my life entirely. I had to make a choice.
My practise sessions started with a 30 minutes warm up made of scales and chord exercises. Then I moved on to existing compositions work, then new work then free research work... once finished, I easily was 4 hours in... and exhausted!!! Looking back now, my practise was too much for my brain to handle and was mediocre at best.
Now, I do this:
Chords and scales: in my head, away from piano visualising them, including chord progressions and other such things. When I start a session, it now only takes a few minutes, 5 at the most because my brain is in full control of my fingers and what I want to achieve.
I do the same with existing compositions. You need to find your own way of practicing them away from the instrument. As a pianist, I remember the fingering sequences: if I play a I chord in C, I don't think C-E-G but 1-3-5. I also "play" of the piano with mental visualisation. This means I can transpose in any keys! Back on the instrument, I practise at half speed, once only unless there's a problem. This means I can do a few existing compositions memory practises in about 20 minutes. I also use that as warm ups. That leaves me plenty of time for new stuff, but again depending on the piece, I only learn a few bars at the time. That way, I usually have 2 or 3 new pieces on the go.
When I practise existing pieces, I never do it 2 days in a row. I let my brain settle down and process it into long term memory.
I also practise sight reading 15 minutes a day with any music book.
It's important to know that I spent a long time, at the beginning of my journey, for practise of music. Scales chords.... I know now it gave me very, very strong foundations.
Something to think about.
Lastly and unfortunately, spending time with an instrument is the real only way to become great but hopefully this answer will give you ideas to spread it out a bit.