Most NAS don't have the processing power to transcode files.
The DS716+ can transcode 4K.
I was looking at getting a Synology Disk Station 2-Bay Diskless Network Attached Storage (DS716+) to stream content to my AppleTV.
Assuming you have a

TV4 there's also a DS Video app in the store, besides Plex etc.
I'm in the same boat, & TBH it's a feckin' nightmare trying to figure all of this out, Synology are similar to

in more ways than I thought!
Up until recently I had no knowledge of anything NAS, RAID etc, then the DS716+ was launched & i thought excellent time to buy, coinciding with rapidly running out of space on the DAS's I already have, so i started reading up on it.
I like the 716 specs & that it's expandable with a DX513.
After checking it out, RAID 5 appears to be best format, but this can't be done with only 2 drives, the last thing I want to do is start with the 716 & 2 8TB drives & then have to re-back up / reformat when I get a 513.
The DS815+ is the most expandable (8 drives plus takes a further 2 x 513 units) but lacks the horsepower according to reviews on transcoding (although spec wise says it should be fine for 1080), combined with no 4K to 'future proof'.
There is the models introduced this year to replace the 'play' versions, such as DS216, DS416 play, but these are no longer expandable like earlier versions, so what you buy is what you're stuck with.
You also have the DS2415+, which although expandable (at 12 drives maybe take a while tho) with no 4K & a bit pricey to fill also.
I know at some point there'll be an upgraded 1815, but if the past is anything to go by this won't be till next year, I could buy another couple of externals & hold out, but fear they may cripple expandability in the same way they've done this year, a 4-bay 716 would be an ideal alternative, but no idea if that'll come.
I had dabbled with the idea of getting an external DAS / RAID enclosure & combining that with a mini to PLEX it up, as the only reason I'm looking at the Synology is to remote access on iOS via the app & i think this'd be the easiest option, but I am open to suggestions if anyone has any ideas?