I have watched lots of your videos on the DS920+ (as you don't have some of the Plex and video station review reviews for the DS1520+) and the DS1621+trying to figure out which one is the best for watching movies and dealing with bigger, denser files, including 4K HEVC. I can't seem to figure out which NAS will be best. While the Celeron chip on the DS1520+ seems like it would be better as it has a little GPU capabilities on the CPU, the AMD Ryzen is a beefier chip and seems to come off better on your videos even though it is doing software transcoding on HEVC.
The price difference between the 2 NAS's isn't an issue (happy to pay more for the DS1621+). But which one is the better NAS for video playback (Plex and Video Station), especially at 4K HEVC?
Also, if I go for the DS1621+, would a RAM upgrade or NVMEe SSD for cacheing help with video playback?