Testing my Maksutov again, but then while I was waiting for the sun to get into a better position without aperture blocking, cloud rolled in.
First one was taken at prime focus, i.e. 1500mm:
And then another closeup at 3000mm:
For comparison, I also captured a 2400mm version with my Ranger and 5x Powermate:
So there seems to be nothing wrong with the Maksutov now. Of course, I will need to test further under better sky.