If you know anyone with a VR capable PC, then go round there and try your headset on their PC. That would at least tell us if it is your machine that's at fault.
It’s also possible to get the experimental version directly from Steam, although perhaps a standalone one is best in this case, to avoid any possible steam issues
Played around with ge.quality settings in GifEncodeTasks (at 1 and at 30), no significant improvement or change to file size. I suspect with the current encoder there isn't much that can be done. Unless a better encoder is available. At this point, aside from maybe taking a look at the other two modes am probably going to explore using a square frame video (?possible?)