Small library, tight budget: the DXP2800. Cheapest real transcoding in a NAS.
One box that stores and serves, the default answer: the DXP4800 Plus.
Big library, VMs, or one machine for the whole lab: the DXP6800 Pro, or the DXP8800 Plus for eight bays.
You already own a NAS you like: put a Beelink EQ14 next to it and let the mini PC do the transcoding.
You’re set on Synology: buy it for DSM and the backup tooling, not for Plex, and plan around direct play. Whichever way you go, the drives matter more than the RAM: use CMR drives, not SMR, and my NAS hard drive guide covers the ones I trust