Before you start throwing money at the problem, let's first determine why you are getting egg shaped stars. You mention the stars have shifted from one frame to the next. This is absolutely not a problem. In fact, we will intentionally induce drift between frames. It's called dithering.
You can't look at the final stack to make this determination. You must examine individual subs. Are the stars round in individual subs, or are they egg shaped there?
Also, how exactly do you have your guide scope mounted to your rig? Is it mounted a stalk that fits into the place meant to hold a finder scope?
I re-read your original post. I see you are getting egg shapes in individual subs. So this does point back to how is your guide scope mounted.
Thanks for looking at it from the money perspective :-). I get your point, but I did mention that I (deliberately) did not dither for testing reasons. Let me explain what I did, based on an advice of a more experienced AP-photographer. I took the first and last sub of my imaging session (undithered), then entered them in the process Pixelmath in Pixinsight, and the combination of the pictures showed shifting of the stars. The other guy who advised me had the same problem (no round stars) with a guide scope, then bought an OAG, did a (undithered) imaging session with it, and entered the first and last sub in Pixelmath: the result showed absolutely no shifting of stars. So he fixed his (flexure) problem with an OAG.
The odd thing is that when i did a second test with Pixelmath today, but now with the first sub of the session and a sub from the middle of my session, again the result showed shifting but this time in a completely different direction (which could not be caused by the telescope position).
My guide scope has 2 guide rings from Primalucelab. Both rings are attached with two bolts that attach to the main scope rings. This look very solid. Possible weak points ar are the plastic contact points on the guide rings, and the soft material on the inside of the rings of the main scope, which might allow some play.
My train of thought was A) with an OAG I could check if it's flexure and B) they are not that expensive.
IMO, with my setup, I should at least be able to get round stars in 1 - 2 min subs. My last imaging session my RA RMS was 0.18 on average and my dec RMS was like 0.13. Is that good enough? So basically I am really happy with my mount performance, but it doesn't show in my images :-)