Go with a 16TB WD MyCloud Pro (?), maybe it's called "Dual". That will have 2x8TB HDDs (like you asked about in OP), and I believe (check their manual), that it can do Android backups. That will probably set you back $700.
Edit:
www.newegg.com
My bad, it's only $499. But it doesn't back up the whole phone, just photos and videos from phones, and PCs and Macs.
Edit: Nota Bene: You generally, want to AVOID single-bay/single-drive NAS units, because they have a single point of failure, one OOPS, and your data goes bye-bye, generally-speaking. At least with a 2-bay/2-drive mirroring (RAID-1) arrangement, if one drive dies, you have the other one, and can just swap in another drive and it should re-mirror the contents.
I also want to plug QNAP and Asustor (have no experience with Drobo or Synology). Most of those are sold "diskless", and are not cheap, but have a MULTITUDE of features, with a very easy-to-use web interface. (Some have phone "apps" too, though I haven't used those.)
Asustor's OS is a bit rough around the edges still (they are one of the newest vendors to offer NAS units), but QNAP's is full-featured. Try to get a model with an Intel CPU in it, and not some ARM or Marvell SoC/CPU, because those tend to be slower, and more limited in terms of OS upgrades and updates.
For QNAP, I would recommend a TS-451+. For Asustor, probably one of their newer "Nimbustor" models, with the dual 2.5Gigabit Ethernet jacks, for future-proofing. They also have some "big daddy" units, the "Lockerstor" 8 and 10 models, with 8 and 10 bays respectively, both with DUAL 10GbE-T and DUAL 2.5GbE-T. Talk about connectivity! Intel newer / newest Atom CPU quad-cores in them. Maybe you can also get them with i3 and whatnot. I think that they also include slots for dual NVMe M.2 SSDs, for caching / tiering. (QNAP OS also allows SSD caching / tiering.)
Also, another benefit of using an Intel native CPU, is support for Intel-compatible OSes in Virtual Machines, running on the NAS. That's a fairly advanced subject, so I won't get into it now, but you can have Windows running in a VM, on your NAS, that you remote into, using a web browser, on any PC in your LAN! Pretty neat, but takes a while to set up.
Edit: Also, some of them, have HDMI or HDMI 2.0 output ports on them, you can hook them up to an HD TV / 4K UHD TV, and play back media, movies, photos, audio, etc., over the HDMI port, or run your VM that way (plug a mouse and keyboard into the NAS into the USB ports on the back). Most also let you stream media using DLNA support over the network to a "Smart TV" or "Roku" device. (*Also "Chromecast" maybe?)
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