You’re really best off not trying to lump together your Plex server and your NAS on the same box. You can use an odroid H4 or any cheapo Alder Lake-N laptop for your Plex server, and any NAS solution to store the media.
Tbh another archaic idea is that you need to transcode.
All you need is one of those sticks that plug into the hdmi of your tv and can run vlc. I use a chromecast. Vlc can play mkvs directly off samba shares. Done.
You got different replies from others, but my 2¢ since you were replying me - if you're not doing transcoding, then I agree, Plex on a NAS is fine - it's basically just a wrapper around some shared files in that case. (The post I was replying to did specify that they were doing transcoding.)
Even if you do transcoding, it's still fine so long as it has supported hardware for HW-accelerated encoding. Which doesn't require all that much - even fairly basic Synology models can do H.265.
Some do, but the models with HW-accelerated transcoding have been shrinking - it's only the "prosumer" models that have Intel Quick Sync (with no guarantee that refreshed products will maintain it) - the lower end models have ARM processors without HW-accelerated transcoding and higher end models have Ryzen processors without GPUs.
