Basically all 8K material in VR is upscaled native 5K/6K.
Try downloading the material with the highest average bit rate and see if you see any difference. Some studios release videos in 100+ Mb/s as an alternative download to the optimized version in e.g. 40 Mb/s. These videos are often huge so expect a 30-60 GB download instead of e.g. 10-20 GB. High bit rate will improve image quality in videos with a lot of movement or changes between each frame.
Even your system might struggle with a Pimax 8K. I’ve got a friend with a similar setup, even better actually, as you have and who is also extremely tech savvy. He always complain over shitty drivers, firmware and configuration software for Pimax and that his rig is barely enough for running Pimax 8K with full FoV. He mostly play VR games though.
In case it’s on the edges you experience bad image quality it could be an issue with the extreme FoV. Lenses combined with displays set at an angle makes perspective/projection difficult to manage.