I usually don't do this since I am seeing the same image (with JPEG artifacts) that you also already see. But...
Notice that the stars on the top left corner are slightly "non-round." Except for some chomatic aberration (typical of non-premium optics even when they claim being "APO"), the top right looks mostly OK. Bottom right looks better. And bottom left is similar to top right. My guess is that you have s slight left-top-bottom-right tilt -- i.e., a small tilt along the bottom-left-top-right diagonal axis.
If some corners look good and some corners don't, it is usually a tilt problem. If all corners look bad while the center looks good, it is a field flatness problem.
Remember that stars are never a "pin-points." Even perfect (finite sized) optics will obey the Dawes limit (i.e., diffraction limited). Their intensity profile obeys a Bessel function in 3 dimensions -- similar but not precisely the same as the familiar sin(x)/x profile in 2 dimension processes. The "tails" of the profile is very long and brighter stars will therefore look larger.
On top of that, atmospheric turbulence ("seeing") will also bloat your stars.
This is what I can do with my FSQ-85 (4-element Petzval + 1.01x flattener, ASI2600MM, 45mins of integration)
http://www.w7ay.net/site/Images/Lagoon.png
You can see what I'd described above -- brighter stars appear larger. Seeing was OK but not great that night (July 24 2021). My notes say HFD of 2.12 pixels (3.59").
(Filter was a little weird, while I was waiting for my Chroma narrowband filters. It is a stack of a "color" Radian Quad filter with a red Antlia filter to pull only the H-II side from the Radian Filter passbands.)
Chen
