Hi Salgo60,
Image Orientation
Yes, though my ThetaX camera doesn’t have a Horizon Lock feature, so I have to do that the old fashioned way* - by attempting to ensure that it’s leveled at all times 
.
Though it does have auto orientation, so (as @Hol_Ger mentioned above) the orientation when the the recording button is pressed does matter in my case; (there have been several occasions where I accidentally started the a time delayed recording at close to 45° angle and corrected the lean and then noticed failures processing failures, as the camera had locked in the rotation and tagged the recording with a 90° rotation…)
I believe this can usually be corrected in post processing by using the “–orientation=0” option when processing the sequences for upload, but once uploaded there are no tools in the app to manually correct the issue.
Position accuracy
Most GPS receivers can only only produce one position fix per second, so anything in between these point usually has to be interpolated during processing. So yes you really do need to mind how quickly you change direction; the slower you move, usually the better the accuracy of the location.
Further, the GPS in my camera doesn’t like being moved around randomly, so when holding it on top of the selfie stick, the accuracy was not good. Most smartphones have the same issue, e.g. if you hold it in you hand, the small random movements the precision can quickly degrade from less then 3m to more than 15m. In my experience placing the receiver it in a pants pocket while walking provides better accuracy, then holding it in the hand while standing in the same spot, though still not nearly as good as when pressed against the upper-body/shoulder. (The double selfie-stick above seems achieves similar reduction in random movement as well.) Vibrations through, seems to have less of an affect; while bumps can make the GPS severely overestimate the actual movement. (Tall buildings nearby can also affect the accuracy quite a bit.)
Thus if you are unhappy with the accuracy, I would recommend also reviewing how you are managing the GPS tethered to the camera, as that could help improve the location pression significantly.
GPS track usability
Regarding the original question about the GPS track usability - 5 points per second is more than most of us get, so if saved as a .gpx file it should be usable. That stated, Mapillary recommend against using a separate GPS track, as it can be quite error prone to align the timestamps in the GPS-track with the time stamps with the images/video.
(Yes, it seems simple, but also in my experience it is very easy to get the offset wrong without noticing in time… so I personally view it a as a last resort, to be used only when the it in not practical to redo the image-sequence/video-recording with the correct GPS position data embedded.)
/Bruno
PS.
Orientation stabilization
For walking I tried using just the Theta TM-2 stick, but the results proved unsatisfactory. Adding a second insta360 sick below, and a 200g weight at the bottom improved the stability significantly, and ensures the stick will be hanging plumb unless it is pushed into an angle. Drawback is that the long stick becomes fragile and can easily be bent…
Secondly the TM-2 stick has a fixed relation between the bottom of the stick and the camera - as there are only two ways the camera can be mounted, and this helps in visually confirming that the camera is consistently pointed in the direction of movement, while held well above the head.
DS.