Well finally I have got around to pickup a pair of RS3614RPxs+ models each with their own expansion unit. A total of 48 bays ;). I will be using a single RS to begin with, stariting with 12x4TB WD gold 7200rpm drives.
These two RS models will be getting a new home soon (after I move to my new place) in a brand new rack that I'm still shopping for. At the moment I have DS918+ as my main box with DS412+ running as a backup.
The idea behind these two RS are pure storage needs and Docker hosts. Not that 918 can't pull its own, but its primary task will remain Plex.
Managed to get some ECC RAM as well to boost them both.
Guessing in the end it will be a fun to have 2 more Synology NAS devices to play around with and offload 918 a bit.