Fortunately that’s not how it works when you have all the access points controlled centrally, like with unifi. Yes there is limited frequency space, but this is much less of an issue on 5ghz. There will also only be one ssid, and handoff between access points is pretty seamless.
3-4 is probably overkill, but the attenuation situation sounded pretty dire. I run two in my house for redundancy mostly, just standard American stud and drywall construction.
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.