But fear not, because transcoding isn't necessarily required depending on how you're going to be watching these files.

Note that almost none of the NAS support 4K HDR transcoding. Plex have provided a detailed spreadsheet of the capabilities of the major NAS' out there. This means that transcoding is often not possible at all, or can only be done on lower resolution files. The CPU in pretty much every NAS is a low-end CPU (often ARM, not even Intel-based architecture). This is actually possibly the single most important factor when considering whether a NAS is what you'll need or not.

TLDR is that you should sell your Mac Mini (they hold value well, mine went for $350) and buy a Synology DS418play or higher. So here's all the legwork I did and some details on most stuff you'll want to know. Isn't is simpler to just buy them already translated English version in the first place, or pay the translator to translate it once and leave you with the English vesrion.Strangely enough, I was actually in exactly the same position as you recently, even down to the Mac Mini 2012 model with a 4TB external drive. Real time transcoding is like buying a whole load of books written in French, then employing someone to sit are translate them into English every time you want to read one. If you eliminate the need to real time transcode, then you can save money on a less powerful NAS (not to mention, noise, heat and electricity whilst it's doing it.) For basic media streaming and file serving, (which are essentially the same thing,) almost anything will do.

In order to maintain the transcode in real time if the NAS cannot 'keep up,' either the PQ or the bit rate will have to be sacrificed, possibly both. Real time transcoding will take as much horsepower as you can afford and still won't be enough. capable of real time transcoding every time you access them. If you only have 'a few' videos that need transcoding, then you'd be better off to transcode them offline, once, using something like HandBrake than buy a NAS with CPU's and/or co-processors, etc.

Cheap nas for plex media server