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Hey Peter
My below comments are only from my personal experience with an 8" SCT, but my help.
I too had almost the same problem with stars until I upgraded from the asi120mm to the asi290mm. I have the older celestron radial OAG and after going to the 290mm its been awesome at 1280mm FL, yet to try 2032mm. I do also hear the zwo OAG is not well suited for the SCT's.
Backfocus! Gees what a drama I went through. All I can say is if I had known about plate-solving and had my ASIair back then, I wouldn't have spent a painful 12 months wasting good nights and buying all these rings etc. All the numbers you hear out there is "ball-park" start with 105mm, plate-solve (focus first) and that will tell you your FL. The closer you are to your spec'd FL you know your back focus is close to correct. I only just work this out when my ASIair told me my FL was 1276mm after a plate-solve and my physical setup was 1280mm. This is with a 105mm +/- 1mm back focus.
Also, a 300mm FL guide scope will not cut it. You are going to get flexture when you main scope is 1764mm and beyond. I was using a 280mm 60mm Zwo guider that could never give me round stars due to flexture. I moved to a ST80 (400mm) that helped heaps at a main FL of 1280mm, but still has flexture until going OAG. Also note my guiding was well under .5 at FL of 280mm & 400mm.
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