(Since you mentioned the specific model, I assume you're familiar with most of this, but just in case...)
I'm also using a DS216J from Synology but with two 3TB Red drives primarily for backups. It's connected to my main router via Ethernet and I have a Time Capsule as a bridge for all other devices in the network. On the software side, it's very simple to setup. And as for compatibility, there's options to support both Windows/Unix users if that's your situation. They call their utility DSM and you can take a look at their main page here:
https://www.synology.com/en-us/dsm/6.1
As a media server for videos, you'll probably want to check out what the NAS is geared towards. The J series that Synology makes, for example, is their low end which isn't really meant to serve as a media server if your files ever need to be transcoded in the future - adding subtitles (assuming they're not burned in) prevented my videos from being played, for instance. If you're willing to do the transcoding on a beefier machine and then move it onto your server, then that could also work. As I mentioned before though, I haven't had any noticeable issues with playback lag, though I'm also used to streaming via WiFi instead, so YMMV.
I don't have any experience with other manufacturers, but at least from reviews, it sounds like QNAP is a solid alternative.
(Sorry for hijacking the thread, OP!)