Feel free to ask questions if you have them. I am no expert, but I am willing to try to help if you get stuck.

1. You are going to find people who have done both. A lot of NAS devices run kind of low powered CPUs so separating it out into two devices can get you more compute power than a single device. For example, an old as the hills file bay may cost next to nothing, and then using your “last” desktop will get you a lot more storage and compute than a 1500$ modern NAS, but it’ll take up more space, cost more in electricity to run, and make more fan noise. This is the route I went. A modern NAS should be able to run what you listed though.
2. TrueNAS scale is all about storage, but it lets you also run containers. Proxmox is all about virtualization, but you can then run a storage solution inside a VM or container. It’s not the kind of thing you’re going to get a right answer for because either way can work. Both are well-documented, capable solutions. I have tried both at times, but I had a lot more experience with Proxmox by the time I deployed TrueNAS, so I stuck with Proxmox and use a TrueNAS box (bare metal) for backups. It really is a matter of preference.
3. If you have a MiniPC and NAS as separate devices, you will want to set up a network share, so you can seed on the MiniPC the copy that’s on the NAS. My seeding, Jellyfin, Plex, etc, all happen in a virtual hard drive mounted in a separate container from the services. Each of the services "see that drive as a network share despite being hosted on the same physical hardware.

I have a Tiny connected to a startech dual USB drive dock. The drives get warm, but not deadly hot. Moving big files is a bit slow, but for streaming on Plex and Jellyfin it works fine.

I didn’t know anything about docker when I set up my NC years ago, so I ran it as a snap on bare metal. Man, it’s gotten so much better! It used to really suck. Like, simple file transfers just didn’t work half the time, so I’d be retrying the same thing over and over… A few years ago, I literally migrated it from bare metal to a VM, but kept the exact same install. I have so much crap on it now, I think I’ll never bother switching it out to docker, just because of the inconvenience. I know the snap version can just run using a local hostname, you just have to set it in trusted domains setting. Might be the same in the docker image?

I used to work on a support desk for a big company. I had a bunch of friends in similar roles at other companies. Fun fact: there are no good support desks. The incentives are all skewed. If a support desk makes the customer happy, it’s giving refunds out. If it’s giving refunds out, it’s costing the company money. If it’s costing the company money, someone in finance or accounting will “fix the problem.” In the case of my company, the fix was: 6 minutes per call, no more. Every call is a sales opportunity, no exceptions. So, people would call in with big, complicated problems, and before I was allowed to help them, I had to try to sell them shit, which pissed them off, got them ranting. This ran out the clock, so then we “accidentally” disconnected. Leather, rinse, repeat. Now, my company sucked, but even the best companies need to limit churn and refunds and deal with a lot of entitled assholes on the support line, so it’s never going to be a great time.

I use Gpoddersync to keep my phone and tablet in sync, but I have one podcast that keeps glitching out and redownloading over and over. Not sure why.

I got an N100 SZBox for cheap a few years ago(?) That has a big USB HDD drive plugged into it. Handles 1080p in house just fine, and I think the bloody hard drive eats more power than it does!

A couple of quick things: in UEFI, keep separate Linux and Windows efi partitions. Disable quick startup in Windows, as it “locks” any NTFS drives you touched in Windows before you booted back into Linux. (I always forget this and then wonder why a drive is read-only… Grr!) Don’t try to install Linux Steam games to an NTFS drive, it doesn’t work. Also, Windows and Linux have different default ways of dealing with the system clock. You can either do a registry entry edit to fix on the windows side, or there’s a Linux fix that is also quite easy, but I forget what and I’m lazy. This is purely optional, but I like to set up grub customizer and set it to boot into “last selected”, so when I’m updating Windows and it restarts, I don’t boot back into Linux, and vice-versa. Also, don’t try to run Windows installed Steam games. It doesn’t work. Lastly, if you virtualize using VMware, your VMs have to “belong” to one host OS or the other, or you’ll have no end to bugs. Personally, I wouldn’t use VMWare on the Linux side at all, except school requires it.