Well, unfortunately no specs have been deployed by Google.
Their specs about suggested settings are limited to 4K.
It’s my option that HEVC/H265 is the best chance for 8K, just because the most of smartphones allow only HEVC for 8K.
In the truth designated codec for 8K and 16K is VVC/H266, but it’s not available on the most of devices, yet.
With HEVC 8K bitrate is 50-80Mbps, maybe this is too much for YouTube.
Forgiving for a while bitrate and quality problems, how did you succeed in uploading 8K?
From web or app? If web what browser did you use?
Computer or smartphone/tablet?
Windows, Mac or Linux?
Regards