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.
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
It depends how valuable your data is, what backup strategy you have, and how long you're prepared to wait to get access to your data when a drive fails.
Personally if/when I migrate my main dataset to SSD, I'll stick with RAIDZ2/RAID6.
I came to Arch for the customisation, I stayed for the AUR
The way I justify self hosting is that every device I use it on has an offline backup so downtime isn't overly important.
Arch, nothing beats the availability and ease of installing packages from the AUR
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.