First let me tell you about one of the apps you mentioned. And how, after you use it, you likely won't want to use DLNA as a protocol for watching media.
Plex has client apps for most, if not all of the devices you listed. Each of the client apps can be installed directly and can use the media stored on your NAS pretty easily, with (hopefully) little or no RT transcoding. (More on that in a bit.) Using these client apps means you won't NEED to use DLNA. (This is a good thing, trust me...)
DLNA can interfere with tracking watched status of a TV show, episode or movie, so if you pause the show you are playing, it can hold the pause, but if you STOP playing it and want to come back at a later time, you need to scrub through to where you were. This is an inherent issue with the protocol. Plex doesn't use this protocol normally... In fact, if you have a Plex Pass, their premium level, and enable the Plex Home, DLNA is disabled, as the protocol doesn't use Plex's user ID's for tracking watched status. When you watch S01E01 completely, DLNA wouldn't automatically go to S01E02, you had to manually select it to play the next in sequence. Totally not acceptable when either binge watching or trying to stay caught up on new shows on an existing series, IMO.
If you look in my signature you will find a number of links both on these forums and on Plex's forums where I've made a number of posts on how to optimize an Asustor NAS and your media with Plex. (I go by the handle MikeG6.5 on Plex's forums.) I'm sort of the resident expert on making the app work on NASes having installed and run it on three different models now. (AS-202T, AS-304T and AS-7004T)
One of the first things you need to do, is get your media into the best Direct Playable formats, codecs and bitrates you can. You want all of your local watching to be Direct Play, and only use Transcoding for remote sessions, and then only in dire cases. The CPU in the 320x model isn't really a full blown CPU, such as what's in the 7004T. As such, it hasn't got a lot of horse power, which transcoding requires... (There is a HW Beta that can use the GPU or Quicksync with intel processors, which provides better transcoding abilities, but there are some issues with quality, especially on really high bitrate scenes...) See my post in the Tips and Tricks section for how to get that working on an Asustor NAS. I have a few guidelines for making this happen in the links in my signature, too....
I tried running DLNA apps when I first got into media streaming and was severely disappointed with the results. When I had over 1000 movies and TV episodes combined, the DLNA clients wouldn't find a lot of them. So even if I knew they were there, the only way to get to them to watch them was to use a folder view. And now, with 2400+ movies and 7500+ TV episodes there is just no way I would be happy using the limits I had hit before....
Not going to long wind this anymore. Most of the information I have on the topic of Plex can be found either in my signature or by searching my user name on either forum. (over 2300 posts on Plex's forums BTW, so a lot of good info, some ranting and some opinions on that forum...) Feel free to PM me if you need more help. I can even be talked into TeamViewer sessions from time to time, if you really get stuck. Most of the time I do it for free, but if you're willing to donate, that's cool, too...
EDIT: It was because of my never ending efforts to make Plex work well on Asustor's NASes that the powers that be decided I might make a half decent Moderator on these forums. I hope I can live up to that expectation.