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.
Sand, sand, and sand some more. When you think you are done sanding, sand some more.
If you have a free pcie 4x or higher slot, you can throw in a cheap card to adapt to m.2 nvme, like 12 bucks. I'm running one in my older hp desktop that doesn't have m.2 and it's been working great.
For a quick test you could set it to 777, if that fixes it, check which user new files are being assigned to, adjust permissions back down accordingly.
We need more info on your permissions. 755 would mean anyone can read files there, but only the owner can write. If the owner isn't the same user that mediawiki is running as, then uploads won't work.
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.
No, the browser would still send YouTube.com as the host header. While yewtu.be could be configured to allow this to work, the TLS cert would not and the browser would get upset.