I have no way to test this with the equipment I have, but what about opnsense on an x86-64 box and throw an sfp+ pcie card in there. You could then in theory turn off auto negotiation and set it to 2.5g. Has anyone out there tried this?
I’ve been running opnsense with my CenturyLink 1g setup, though I’m still using their ont to convert to copper, and been very happy with it.
Jellyfin is very conveniently packaged in docker, so while it may seem daunting, I highly recommend at least trying that route.
Running an nfs mount, docker or not, should be perfectly fine. Jellyfin just uses normal storage so won’t care if it’s nfs. No real special considerations with proxmox either, especially without worrying about a dedicated GPU. Just spin up a Debian guest and go.