I personally use Unraid and couldn't recommend it enough. Can easily do what you need.

I, like almost anyone else here, will recommend linux. I’ll get back to that later.

Your hardware setup is… fine. Plex has pretty low hardware requirements and runs well on old hardware. The real hangups are memory minimums and the ability for hardware-based transcoding. The second you have covered with a CPU with QuickSync enabled. The first: you’re going to want upgrade to 16GB when you can, but 8GB will get you by for now. The memory is less for Plex itself and more for the stack of companion apps you’ll want to install alongside it, the minimum of which include:

• Sonarr (for automating tv show downloads)
• Radarr (for automating film downloads)
• Prowlarr (enables multiple trackers and forwarding search and results to Sonarr/Radarr)
• a torrent client (transmission, qbittorent, etc.)
• VPN client container(s) (this setup depends on your VPN and the setup may range form ‘easy’ to ’nightmare’ depending on the service you use)
• Docker, to run all of this in containers (including PMS itself)

(see the c/Plex add-ons guide for more info on these app and links to setup guides)

Now to you choice in linux distro: In the past, I would have recommended Ubuntu, but it’s been on the outs lately since they’ve switched their software distribution model to Snaps rather than the industry—standard Flatpacks, making life difficult for some people. Instead, I would recommend Debian (what Ubuntu is based on), as it has improved dramatically in usability and ease of setup. And software that works/is built for Ubuntu works for Debian, so you’ll have a wide breadth of available software packages. It’s a 30 year-old distro that’s stable, reliable, and very well-known, so it’s not going anywhere and has tons of community support. It makes for an excellent server OS.

One alternate I’ve seen people mention is Pop_OS!. It’s based on Ubuntu (but uses Flatpacks) and has a slick, shiny, custom version of GNOME, but, as a server OS, offers little advantage over, say, Debian. It’s biggest advantage for users is their proprietary NVIDIA driver solution which is the only one i've seen work out of the box on linux, but that’s not an issue for you.

I’d stick with Debian if I were you. It’s simple, easy to set up (relatively), and you’ll have zero trouble finding all of the software and support you need in abundance.

I hope this helps. If you need any more support or have any questions, don’t hesitate to ask!

ADDENDUM: your data access speeds to your HDDs is not a real priority. Streaming the data from your drives takes little data bandwidth, so while it would be nice if all of your drives were internal, it’s not necessary. Many people use external, NAS, and even cloud storage solutions without issue. The main issue is capacity. You’ll use that up pretty quickly and also quickly notice that you’re buying new hard dives often to meet your demand for more capacity.

On that note, arrange your media storage (not your boot drive) into an LVM (Logical Volume Management) drive pool solution where your drives are a unified storage pool to which you can simply add another drive when the storage pool runs low on space. LVM basically treats a group of HDDs as a single drive, distributing the data evenly across them. And, if one drives started going bad, the LVM software can shift the data off of it so it can be removed from the pool (ideally onto a new drive you add first).

Open media vault is the distribution I would use. No gui install at all. Everything can be managed in a web browser on another computer.

You will have more than enough ram for plex. Or jellyfin if you ever decide to give it a try. I run both. Plex has the advantage of being really simple for external access.

Use docker for the install and run plex in a container. Its easier than you would think. Lookup plex install docker or docker compose. Open media vault supports containers now. There are videos that can show you how.

As for the cpu. The biggest benefit is likely power savings. I upgraded recently from an old cpu to a newer one recently and went from 60 watts to 10, AND passive cooling which means it's near silent.

I personally use Proxmox on baremetal and just run a bunch of VM's, most are Ubuntu. But with only 8gb of RAM, a hypervisor might not be of much value to you. But if you ever upgrade the server and want to do more with it, having a setup with multiple VM's will be really handy.

Otherwise, I'd say go with Ubuntu or Debian on bare metal. I personally don't like the NAS style distributions, I prefer to setup everything myself, rather than getting a distribution preloaded with a bunch of shit I'm not going to ever use.

I'm no expert but I'll offer my input until one shows up.

I use Ubuntu for my Plex server. It eliminates a lot of your complaints with Windows. It's less resource heavy, you update it yourself, mine runs 24/7 unless I've done a kernel update which requires a restart, and that's just restart time.

It's definitely among the more ease of use distros, and what you don't already know will generally not take much effort to find or get help with. Running command line only will free up resources and likely won't take long to pick up what you need to run how you need to.

All that said I've not done anything with NextCloud or Bitwarden.