Im seeing about the same thing. Sitting next to the router at about -52 and speedtest showing 110-120mbps but on a 150mbps capped connection. I’m also running narrow 20mghz on ch 165.
Having 2 antennas instead of 3 shows in the signal strength for 2nd floor.
Planning to hook up 2 minis this week. 1 on top floor other in basement with the B20x on the main floor.
I think the upstairs AP mini will probably be enough to cover main floor so will decide whether I shut off the AP on the B20x or not.
What I may do is schedule the AP minis to shutoff at night and the B20x to come on at night. More of an emf thing while everyone is sleeping. But 100 other networks broadcasting 24/7 around us won’t help.