Help me refine my current Orbi home WiFi network (RBK50) (VS change to Nighthawk setup)

>> combination of recommended Nighthawk + Orbi Setup

Setup #1:

Configure the Nighthawk as a 5GHz wireless Media Bridge to the RBR50 upstairs.  This will enable wire access from your Nighthawk to the media server as well as the main computer downstairs.  Assuming the Nighthawk is capable of gigabit wireless, you should get reasonable speed from RBR50 to the Nighthawk via a 5GHz Media Bridge.  To keep it simple with single SSID, disable the 2.4GHz radio on the Nighthawk so that it will always connect to RBR50 via its 5GHz radio in order to get maximum speed.

If the Nighthawk is not up to the task, try this:

Setup #2:

If your Nighthawk can't handle gigabit wireless or you want super fast wireless in that room, move the RBS50 there since it has a powerful 4x4 wireless backhaul and it will have no trouble punching through that ceiling to reach the RBR50 (Put the RBR50 on the floor will also help).  This will allow you to gain wire access to all your devices as well as fast wireless from RBS50.  Since you can run cat5e cable to the current RBS50 location, setup the Nighthawk there as an Access Point by connecting the cable from RBR50 to one of its LAN port.  To get wireless in that room, setup the same SSID as RBR50's SSID, but pick different 2.4/5GHz channels to avoid collision.  For example, force RBR50 to use channels 6/36 and the Nighthawk to use channels 11/40 for 2.4/5GHz respectively.  This will ensure that both can coexist peacefully.

Setup #3:

This is simplest of all, but costly because you will have to get a second RBS50.  Put one in each room.

I had similar problems and wireless Media Bridge + Orbi WIFI combo resolve my slow or no wifi problem.

YMMV, try it out to see which one works best for you and stick with that one.  Good luck!