non-free non-free-firmware allows vendor none opensource firmware and packages to be installed.I tried switching to `deb http://ftp.uk.debian.org/debian bookworm-updates main contrib non-free non-free-firmware` and this updated pve-firmware. The ethernet has now been up for over 24 hours... this might have been the fix! I'm not sure what the non-free non-free-firmware does but it does look like that was what was needed.
so basically you now have the vendor firmware (realtek) on the device rather than whats in the mainline kernel tree which could be well outdated.