I went with the DS1621xs+, the main driving factors being:
that I already had a 6 drive raidz2 array in truenas and wanted to keep the same configuration
I also wanted to have ECC, which while maybe not necessary, the most valuable thing I store is family photos which I want to do everything within my budget to protect.
If I remember correctly only the 1621xs+ met those requirements, though if I was willing to go without ECC (which requires going with xeon) then the DS620slim would have given me 6 bays and integrated graphics which includes quicksync and would have allowed me to do power efficient transcoding and thus running Plex/jf right on the nas. So there’s tradeoffs, but I tend to lean towards overkill.
If you know what level of redundancy you want and how many drives you want to be running considering how much the drives will cost, whether you want an extra level of redundancy while rebuilds are happening after 1 failure, how much space is sacrificed to parity, then that’s a good way to narrow down off the shelf nases if you go that way. Newegg’s NAS builder comes in handy if you just select “All” capacities and then use the nas filters by number of drive bays, then you can compare whats left.
And since the 1621xs+ has a pretty powerful xeon, I run most things on the nas itself. Synology supports docker and docker compose out of the box (once the container app is installed), so I just ssh into the box and keep my compose folders somewhere in the btrfs volume. Docker nicely allows anything to be run without worrying about dependencies being available on the host OS, the only gotcha is kernel stuff since docker containers share the host kernel - for example wire guard which relies on kernel support I could only get to work using a user space wire guard docker container (using boringtun) and after the VPN/tail scale app is installed (presumably because that adds tap/tun interfaces that’s needed for vpn containers to work.
Only jellyfin/Plex is on my NUC. On the nas I run: