If not, run Plex Media Server on a PC running Linux and leave the media on the NAS. If space is a consideration, get an Intel NUC or similar small form factor computer.
This gives you the potential of a much more powerful Plex server, with an i3/i5/i7/i9 CPU versus a Celeron.
Regarding your NAS list, mark off the DS1522+. It has an AMD CPU, so no hardware accelerated transcoding with Plex.
I upgraded to Synology DS920+ and am very happy with it. I had my PMS on DS916+ before and was running out of space. Decided to throw in for new NAS and DS920+ seemed to be best all around for PMS. It plays 4k content without issue within and remotely from network even when transcoding.
UPS will be delivering a QNAP TS-664, (2) 1TB M2.SSD for the OS/Apps and (6) IronWolf 16TB drives for storage in RAID 5.
Went with the QNAP ultimately because of the specs, and perceived QNAP saturation here in the forums. I intend to install and crash it 4 times in the first 2 weeks before I get serious about the Plex build. I need to be comfortable/familiar with the EMERGENCY REBUILD procedure before I go all in. LOL
For those who run a QNAP ANY AND ALL tips will be appreciated.
Final note on this adventure. As I said, I purchased a TS – 664 from QNAP Systems Inc. I can say that on the Plex side it is flawless! Plenty of power, transcoding is seamless. Hi-res, lo-res, multiple streams, etc. I am thoroughly impressed. There are some advantages that I am enjoying over the ReadyNAS and it was ultimately the correct/upgrade purchase for me.