Hi, Apple user since the Apple ][+ and owner of a (struggling-to-hang-on) independent Apple reseller, one of America's oldest. However, I elect not to run Plex on my Synology as it's cheaper (and more flexible) to offload transcoding to Plex running on an external system, rather than purchasing one of the higher-end Synology models with enough CPU power to handle high-quality transcodes. It's worth noting that Synology devices themselves can run a number of useful software packages (and many models can run Docker containers), including Plex. All the 'server' systems just mount directories stored on the Synology over a NAS protocol (NFS, SMB/CIFS, AFP). In my case, I have a Mac Mini doing some things (mostly transcoding video to h.265 for long-term storage), and some VMware ESXi hosts that run virtual machines doing other things (Plex server, OpenVPN server, virtual desktop, etc.).

My preferred solution is to throw data (Time Machine backups, Plex library, archive of personal photos, etc.) on a Synology NAS (I'd recommend a model with 4+ drive bays so you can capitalize on RAID-5 to protect against drive failure), then use another device for actually handling server roles.

Plex media server mac mini