Currently im only running on System 1, Plex + -ARRs, also TDARR.
I want Plex alone on one system and all other services on the other, but which should I choose?
I have about 30 active users and a huge collection (4600 shows, 9800 movies). So while I see the quicksync of System 2 might do more transcodes will it handle the server itself just as good as System 1?
Probably better. Plex on the OptiPlex, everything else on the 846. You might want to get it more RAM and transcode to ramdisk. Keep the media acquisition (arrs) on Unraid, close to the disks.
How many max simultaneous transcodes? Keep 4k content in a separate library limited to those who can direct play it.
If you exceed the capacity of the 8700T, you might sell the P2000 and get a second QSV transcoder. Run a second instance of Plex on it, mounting the same media, and split up your users.