Stellarmate works like a charm for me on a Pi4.
It would be lovely if I could use my Polemaster on the Pi as well.
The software from QHY for the Pi is written for Ubuntu and I can't manage to make that work on Stellarmate 1.5.3 (Raspbian).
The Polemaster works just fine on the Pi4. Use the internal polar alignment module and select QHY CCD as an auxiliary attachment. It will directly recognize the polemaster.
Then just use with the polar alignment module in the solver tab.
I do it all the time. No other software required. Works like a charm with Kstars as is.
Atlas Pro AZ-EQ, ASI1600MM-Pro, ASI120MM-S, ES102ED, WO-Z61, Nikon D3300, ASI-EFW, ZWO LRGB,Ha,O3,S2 filter set
Also using Stellarmate on a RPI4 , have it mounted on top of the telescope with a USB power unit so totally portable . I am using the Flirc case I had to turn it upside down and all my WiFi issues were solved. The RPI4 is a lot faster than the RPI3B I was using.
How did you fit the RTC I used it with some wires and just wrapped it in some tape . Not much room in the Flirc case.
I am using Kstars on a Macbook Pro with MacOS Catalina and works well mostly but does crash when capturing images/video as you have had the same issue. I did report that problem here also. I tried versions 3.4.3 and also an older 3.4.1 still crashes at times when capturing images.
To get around that I run ccdciel for imaging still using Kstars/Ekos for Platesolving and mount control.
I am using ZWO camera, filter wheel and a AZEQ6 mount.