Why GPU? Just get an Intel CPU with quick sync.

can also handle transcoding

This CPU has quick sync so should not be a problem. I doubt you will need more than 3-4 4k to 1080 transcodes if you are remotely considering this board so it should not be an issue.

My only concern with these one off mb manufacturers is driver support in your os of choice including Linux variants.

You are never going to get 15 Sata on a board. Just get HBA card in PCIe.

If it just adds content to radarr and sonarr, it wouldn’t matter for Plex. Just set to scan folder for changes.