Building KStars via the github instructions did not work well. It needs apt-get-repository that I don't have and there are no instructions on how to get it. It also requires XISF that was optional so far, and StellarSolver. The prerequisites make it a bit overwhelming.
On the Losmandy Hackers IO board I received a very good tip though. Check out gitea.nouspiro.space/nou/astro-soft-build . It is a build-everything KStars/Ekos/indi/ind3rdparty script that performed flawlessly until my Pi crashed. I'm guessing it got overheated.
I'll restart the Pi and see how far it got then try to resume from there.