I think you should have a big project and a small project all the time. The small projects will be short pieces that you can learn within a week or two, the big project will take longer, up to a few months (you said you can't practise regularly).
For the big project take something long such as Schuberts Impromptus or Moments Musicaux. It doesn't ned to be difficult but you want to play this piece without mistakes, with good musical expression and confidence when playing - and it should always feel good (no bad tension, no distorted wrists or overstretched fingers etc.). So this will actually take quite a while but it leads you back to good technical playing.
For the smaller projects, just have fun with easy pieces, try to play them as nicely as you can, analyse the music for more appropriate expression, experiment with wrong interpretations etc. This is where you reactivate your musical instincts, and you build up a small repertoire for causal performances.
Czerny, Cortot, Hanon - whatever you like - just don't overdo it, these things can hurt badly, especially since you are used to be playing much better than today.
By the way: I had a break of 28 years on the keys and now, after 7 months I am far beyond what I could do earlier. Thanks to a good teacher, internet community and a somewhat more mature brain
