I stumbled across this information a few days ago... the fenix 8 uses the Synaptics SYN4778 for GNSS.

www.f-blog.info/garmin-fenix-8-review-but-only-the-internals/
www.synaptics.com/products/wireless/syn4778-multi-band-gnss

I only have data for one test session from the fenix 8, but it looked a little better than the FR-255 on that particular day. I'll look to do further analysis over coming weeks... as and when I get some data. In the meantime, I've added what I do / do not know to my page listing Garmin watches.

logiqx.github.io/gps-details/devices/garmin/watches/

I am just sharing this info now as a quick FYI.