I think you are definitely getting into overkill territory here with a server rack dedicated to Plex and a GPU. Most igpus on Intel chips can transcode 10+ 1080p streams and 2ish 4k streams. Unless you have 100 users seems over kill to me.
Edit. I'm on Unraid with about 30 users, and I only give Plex and all my other containers 6 cores from an i7 10700k.
For all the Julian simps out there, try reading I, Julian by Gore Vidal. Essentially Julian fan fiction written by an all time great.