Why not use a pi or some other sbc?

Check alternativeto.net. Search for any software and it will give you a list of other options and what they are all about.

I have a qnap. I have had no issues. It runs its own qts OS so no need to figure out what you want to run. Make sure the hardware is x86. Plex runs better on x86.

I assume the disco fever is all the pairs shorted together. Is this cable buried or run through the house? I would start by redoing your ends. Make sure they are crimped fully and both sides are identical.

How do you know the cable is damaged? If you have a toner you can hook the generator to the damaged pair and listen along the length of the cable to where the tone changes. It’s not as fool-proof as a tester, but much cheaper.