my $0.02
Both pictures you provided look fairly reasonable. As astrotracer doesn't do rotation longer exposures will show trails as you move out from the center. Also at 260 seconds you really are really close to the upper limit of what astrotracer supports. Instead I would have probably shot that scene as 6 to 8 60 second exposures at ISO1600 and stacked the individual images using Deep Sky Stacker. Between each shot I would have recomposed the image to keep the center as close as possible to the original image. My general rule is to try and keep astrotracer exposures to under 1/4 of the suggested maximum as it seems like the errors really do start adding up.
Your first image looks like it isn't fully in focus, I would suggest getting or
making a
bahtinov mask so that you can nail the focus. I would also suggest going to f/4 to see if that helps.
I have had pretty good luck using the O-GPS1 on my K-3. I have been chasing some deep sky objects and as I am not all that good at finding them I end up with star field pictures. I started without any form of tracking and would just crank up the ISO so that I could avoid trails. The biggest improvements I had were when I did the following:
1. Getting a heavier tripod
2. Using the 2 second mirror up delay with shutter release cable
3. getting bahtinov mask for focusing
Astrotracer helped but that is only because it allowed longer single frame exposures. That can be made up for with a bunch more short exposures at a higher ISO. Try to eliminate other sources of error before throwing more expensive gear at the problem. Also astrotracer goes sideways fast if not calibrated correctly. It went sideways on me Saturday night and I got one frame that had the whirlpool galaxy (M51) in it where the galaxy was just above the noise floor of the image and the in rest something went wrong and I ended up with star trails going in all sorts of directions. The good one was the first one where I was checking that the settings were producing something sane and that the calibration was correct so things went wrong after that. I think it is because the battery may be dying in the O-GPS1 as it is the one I first put in it.