Like the others, I suggest you stick to a distro designed for desktop use (Ubuntu, Fedora etc), you'll have a much easier time.
If you really want to go with something closer to "scratch made" I'd recommend Arch. Its documentation is killer and you can build a system suited to your requirements.
I have a similar setup using Truenas to store data. I've setup a VM in Truenas that can access the data via NFS (easier to setup on Linux than SMB).
It's nice to keep all your services contained in one machine, as long as it has enough resources, and will probably consume less power than running another PC.
I use qbittorrent, most people seem to agree it performs better than Transmission. It's accessible from a web interface.
RAID5 is risky on drives that large, there's a decent chance of a read error during a rebuild.
RAID6 will provide more protection but you lose two drives worth of capacity to the parity data. I'm not sure if a three drive RAID6 is actually possible but a three way mirror would be more sensible as you'll avoid the extra computation of parity calculation.
Imo RAID6 starts to make sense in an array of 5 or more drives.
I just tried adding a native VPN and there's no option to use Wireguard
You should try out KDE in a Live CD. The snapping and tiling features work very well, Windows needs to catch up
Pinhole has allowed custom local records for a very long time now
My method to get around this is setting up a specific user account for my TV. In the user options you can disable transcoding
It is paid for, with your time ;)
Higher end cable testers can show you where the break is, but it will be far more expensive that a new cable.
Jellyfin? It's always supported 4K afaik
And if you're looking for a way to simplify the setup process: borgmatic
I use Arch because it makes installing almost any software package trivially easy via the AUR and if you run into issues, the wiki is there to help.