Me too. Nxmp with plex compatibility on horizon os would be next level. You described it very well, that's all what plex can do. I have a plex server on my nas and I like it very much because it acts like a home made Netflix with your own media files.
I should give a try to it then, I have dedicated debian server acting as NAS and some little things on my network so teorically should can install plex.
My content are mostly anime shows and movies, Kodi handle it fine with TVDB, you know if plex can retrieve the anime infos?
I gona try it now, the main problem of Kodi it's that have their own DB only on each client, then when I gona to try to see my medias on other client than PC, can't see the progress or remain at same time because its on Kodi DB, seems that plex solve my problem there.
The only thing that worry me about, it's that I'm Kodi user since XBOX, means more than 10 years using it, so jump to other client gona be hard, but the good news are that can have both working without problems.
I read that plex can decode and encode videos on the server and serve to clients to make it more smooth, this function are perfect to play 4K files for example on NXMP (theorically the plex server can encode it to 1080 or 720p for the client) but it's a premium function as I read on their webpage.