Thanks for your suggestion, I have played around with all Wacom settings and all affinity settings afaik (not ink, I'm on OSX). in detail:
Wacom pressure sensitity makes no difference
Played with AF view quality, bilinear works best, and Display - open gl works better than Metal in my case
Played a lot with AF brush settings (now I use 90% size pressure, 57% accumulation and 100% hardness)
When I use my settings in any other program like PS or Sketchbook, the lines are fine so my assumption is it is not related to my system settings since it is isolated to affinity photo. If I draw my lines at a certain speed, it is less noticeable but of course I would prefer not to adjust my drawing. I have seen similar issues with other users in older posts so it somehow seems and old issue.