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.

You should try out KDE in a Live CD. The snapping and tiling features work very well, Windows needs to catch up

My method to get around this is setting up a specific user account for my TV. In the user options you can disable transcoding

Higher end cable testers can show you where the break is, but it will be far more expensive that a new cable.