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To me it looks like a combination of things.
1_ you're slightly out of focus so there's a hint of trefoil even on-axis. Maybe pinched optics.
2_ slight elongation in RA so guiding/tracking is not optimal even for 60s.
3_ a bit of field curvature so a corrector spacing needs to be tweaked.
But in all don't beat yourself up. It's still a very good field at that FL. Those are minor defects that can be fixed.
I wouldn't touch the collimation. The field is already symetrical.
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