Well this is a very one-side conversation. Here is my update so far. I have rebuilt my RPi3b+ several times with:
Ubuntu-Mate 20.04 64bit: very slow, almost unusable.
Ubuntu-Mate 20.04 32bit: still very slow, didn't continue.
I didn't try the 18.04, maybe that would have been OK.
Raspbian (most recent release mid-2020): Seemed fastere.
installed rlancaste github installation for Rpi3. Very slow and hung several times. Changed compiler flags from -j (.... -2) to -j(....-1) and then -j(.... -0). The compile finally completed with one error on weather.cpp. I didn't worry so much.
The issue with -j (which is the number of processors to use) was caused by memory utilisation. Even running at 25% CPU utilisation, memory utilisation was 70% at times. Obviously more processors pushed it even higher.
KStars, PHP and Indi seemed to install OK and I can detect hardware and loop photos, which I couldn't before. The RPi3b, seems a little under-powered, but the big lesson was I needed to upgrade Raspbian before I could install KStars/Indi etc. I should probably also buy an RPi 4. But the RPi 3b+ is still good enough. A 240 sec exposure will always take 240 secs, and a faster processor won't change that.