Tim, that all sounds promising in terms of having reliable wireless connectivity. And I agree with Willie/Steve that the router won't have a problem routing between any reasonable number of devices.
If you were to ever find yourself wanting more than the four ethernet ports it provides, btw, the simplest and most inexpensive way of accomplishing that would be with an "unmanaged" network switch. Many models are available at Newegg at very low cost. All you would do is connect the cables to it; no setup or configuration would be involved.
I took a look at the literature on the two NAS devices at the manufacturer websites. One thing that strikes me is that I'm not at all sure that the Seagate NAS is capable of operating wirelessly, through a wireless dongle. There is a very confusing reference to wireless connectivity under "system requirements," but I believe that is intended to mean that for a computer to communicate with the NAS the computer itself needs to be able to connect to the router either wirelessly or wired.
The Synology NAS, as you probably realize, requires that a third party dongle be purchased separately for wireless connectivity. Their site lists a great many suitable dongles.
Regarding USB connectivity, I believe that if the NAS were designed to be able to connect to a host (the Oppo in this case, or a computer) via USB, its USB connector would be the square type, that is used on printers and other peripherals, rather than the rectangular type it has, that is used on computers. And, consistent with GZ's comment, a NAS is basically intended for use on a network, via a router.
Regarding GZ's questioning of the need for a NAS, I think he makes a point that is well worth considering. It would probably be simpler and cheaper to just get two USB external hard drives, one used for playback and the other for backup. Or better yet, get three drives and have two backups.
Regards,
-- Al