Resolution is fairly unimportant if the field of view is correct. The only benefit you get is somewhat nebulous digital zoom which you shouldn’t need if the field of view is set correctly. The downside with 4K is that it generally limits cameras to 10-15fps which is fine in some instances and crap in others.
Your ASUSstor NAS can handle any ONVIF camera so that’s pretty easy. Anything from a major manufacturer will be ONVIF.
And a good night picture needs further clarification. Do you want black & white pictures (Infra Red) of do you need colour? If you need colour then you have Hikvision ColorVu (4MP) and Dahua TiOC (available in 2 MP and 5MP with 8MP actually available in October-ish).
So you probably want one of those two options.
That’s a REALLY long way away. So far away I would probably suggest a zoom camera so you can frame the image nicely. But that rules out night-time colour.
In a fixed lens, Hikvision do 4mm and 6mm versions of this;
https://www.hikvision.com/mtsc/uplo...2CD2347G1-L(U)_Datasheet_V5.5.82_20190429.pdf
and the equivalent Dahua would be this one;
https://www.oprema.co.uk/getattachm...a0-ec3f4fb8a9fc/IPC-HFW3549T1-AS-PV-0360.aspx
If you look at the picture I posted above from the 3.6mm Dahua TiOC bullet you can see how far away 15m looks with even a 3.6mm lens.
A good 4K IR zoom camera would be something like;
https://www.securitydynamics.co.uk/media/assets/datasheets/IPC-HFW2831TP-ZS-S2 Datasheet.pdf
That’s a very good zoom on a reasonable sensor. All the cameras I’ve listed are roughly the same price (£150-ish incl. VAT).
Also bear in mind that a high up mounting location means you won’t see faces very well.