Hello again everyone.
I’ve had a chance to try the basic template FPS, with rotation rate as turn input, on a Galaxy S9+ (Unlocked). The issue was very present and much worse. Interestingly, the issue was so bad that at first I thought the motion input was not compatible, was not turned on, or had an error. Only after rotating the phone quite quickly did the screen jump (No smoothness at all). It would seem that practically all vectors returned on a Samsun Galaxy S9+ are zero vectors, and rotation rate is unusable.
I am going to again try and find someone with a Pixel 1/2 to see if the Samsung VR stuff is the issue or if there is an issue with the Galaxy itself.
I am not sure how to go about the issue of Google’s VR services being a possible cause, as these apps can’t normally be uninstalled/disabled. With that said, I don’t suppose many prospective users would have the same patience as gamers 25 years ago, in that getting the game to run or run properly was the first boss fight. 
Thanks for all the help so far.