What you're looking to set up is quite common in the world of NAS.
Whilst a business would possibly buy 2 NAS at once and replicate them across 2 sites, in a Domestic User / Prosumer environment this typically happens when you buy a new NAS, the old one becomes the backup.
It would logically make sense for both to be the same brand, and be much easier for you to manage, but in terms of capability you can mix brands and achieve the same result but it's not quite so straightforward and intuitive.
There's a whole plethora of out of the box applications, add on applications and even 3rd party applications that you can run on a NAS now.
Your choice will hinge in part on how much 'Plex' media you have (or intend to have), what other applications you are wanting to run and possibly if you'd be happy backing up to an external HDD then buying that second one in a couple of years.
File storage, network shares and remote access are pretty standard stuff across all NAS brands - but QNAP & Synology definitely lead the market in this arena.
QNAP is a more technical, customisable and slightly advanced brand and Synology is more user friendly, marginally less customisable and just trailing a bit. However, they both have models that comfortably cope with what you need, it's at the sharp end the differences start to show.
For Plex and transcoding media files on the fly, you're going to need a higher spec than that backup device. For your backups I'd probably recommend shying away from the most basic devices, just to give you a slightly longer shelf life and a couple of options if you need them later.
So for your backups, QNAP I'd recommend the TS-231k , or TS-231P3 if you need a 2.5GBe port. Synology that translates to the DS218, but no low level NAS with that 2.5GBe port. The closest you can get is the DS220+ with 2x1GBe that you can link aggregate.
For your main NAS, if you're sticking with 2 bay for QNAP the TS-253D is a mighty fine piece of kit that will handle your Plex, Data, and quite a bit more with expansion options and a little future proofing. It's 'baby brother' the TS-251D isn't a million miles behind, so if price is a factor this would be the more sensible selection.
In Synology terms the comparison would be the DS720+ (a little short of the QNAP) and then you're back down to the DS220+, and you probably don't want 2 of the same make and model.
Of course, most of the models I've listed come in a 4 bay flavour as well, just in case you want to take that main NAS up a gear in a couple of years - just add more discs when you're ready, or when there's a stonking deal that's too good to miss.
For HDDs you'll be wanting some WD Red NAS drives (SSD or HDD) or some Seagate BarraCuda's. No need to go for the Pro or enterprise ranges at this stage. But since you already have 4 x 16TB HDDs then you're well away already - just check the compatibility with whichever NAS(es) you settle on. Compatibility isn't 'essential', but I'd certainly recommend staying within what's suggested.
So the rest, is down to preference of operating system. It doesn't matter which model you choose in QNAP or Synology, each manufacturers system is practically identical across the entire range (you just get a bit more with the higher spec machines).
You can try both out here (depending where you are in the world there might be a regional variation):
https://www.qnap.com/en-uk/live-demo
https://demo.synology.com/en-us
Once you've decided, be sure to come back to NASCompares and hunt out one of the many buyer referral links. It might save you a little, but it'll certainly make a small contribution to keep the site running without charging.
Hope this has been both helpful and enlightening, and welcome to the world of NAS.
