You want to stop paying for cloud storage: the DH2300, or the DH4300 Plus if you want RAID 5 and room to grow.
You want a small Plex server: the DXP2800.
You want one NAS that does everything well: the DXP4800 Plus. This is the default answer.
You’ll run VMs or heavy Docker workloads: the DXP4800 Pro for the extra cores and 96GB memory ceiling.
Network speed matters more than Plex: the DXP4800 GT (dual 10GbE), or the DXP2800 GT in a 2-bay.
You’re consolidating everything into one box: the DXP8800 Plus, or the DXP6800 Pro if six bays are enough. Whichever unit you pick, put a UPS in front
Plex serves video two ways. Direct play sends the file to your TV untouched, and almost any hardware can do it, because the server is just moving bytes. Transcoding re-encodes the video in real time, and that’s the part that needs a capable chip. You transcode when someone watches remotely on limited upload bandwidth, when a device can’t handle a codec in your library, when subtitles have to be burned in, or when a 4K HDR file plays on a 1080p screen and needs tone mapping. If everything in your house plays through an Apple TV or a Shield on your local network, you may never transcode at all,
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