That might explain the problem. Assuming adguard returns an nxdomain for blocked sites then the devices will try with their secondary DNS server and get to the blocked site

Your adguard config looks strange. The examples shown list different DNS providers but you have pointed it back at itself for its DNS. I don't understand why you would do that.

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.

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.

Transcoding 4K isn't as hard as you think. I used an i5-9500T and the iGPU could easily transcode ~80GB 4K Blu-ray rips at double real time speed. I've now switched to software transcoding on a 5800x and it also exceeds real time speed.