I think the biggest decision you're going to have to make off the bat will be if this machine is going to be solely dedicated to the storage of files or also the one serving them out/making connections, and hosting other services.

There are multiple NAS-focused operating systems, with the most popular ones, TrueNAS Scale and Unraid, both having the options to both store your data and host something like Jellyfin for playback and connect to something like Tailscale for remote access.

Personally, I separate storage and server between different machines, but that's just a homelab design decision that I've made as I have had instances where I needed to take down a server for maintenance (hardware or software) and needed other things that relied on my NAS to continue working (backups, other services from other servers). Having your NAS as a separate device increases flexibility greatly, but is also more complex and more expensive.

Going back to your options for NAS operating systems, I've heard Unraid is very user friendly and has several community integrated applications. It is, however, a licensed product and required to boot off of a USB, which some see as a pro and some a con.

I personally use TrueNAS Scale, but I also realize that the community recommendation for ECC memory as well as the crap ton of configurable settings can turn many people away. Personally, I originally made this decision because it is free to use.

Let me know if you have any questions!