I have been trying to use the polar alignment routine with my miniguidescope (130mm focal length) and the 5II-M camera since mid-July and never had success. FOV is 2.9x2.3 arc degrees. I can't seem to plate solve with this combination, and I have assumed it was camera noise that dark subtraction didn't correct. I have binned 2x2 and don't recall at the moment what exposure durations I tried. I'm looking for suggestions to see if I can get this to work.
I have been able to polar align with my ASI 224MC on either the guidescope (FOV 2.1x1.6 arc degrees) or my primary scope (FOV 0.6x0.4 arc degrees) so it's not a plate solving issue, per se.
Both PHD2 and the internal guider are able to handle the noise on the QHY 5II-M, and guiding seems reasonable.
On my last outing the ambient temperature was ~15 C (59F) or cooler. I was able to not only plate solve with the QHY 5II-M, I was able to polar align with it. Perhaps there is a noise issue which is really bad in the Summer (~25 C, 77F, at set up / polar align).
I have been using the same setup for polar alignment. However, so far I only tried the sharpcap software. I didn't encounter any problems or noise issues even in summer. I also tried the ekos procedure but couldn't finish it because I didn't have a goto mount at hand (tried it on a star adventurer travel mount). But ekos solved all intermediate images.