I might be reading the thread wrongly (for some reason I've woken up at 6am on a Sunday morning and my brain isn't fully functioning yet) but you're now running 1x 5th gen and 1x 6th gen Airport Extremes?
My venerable Netgear DG834 modem/router does all the DHCP/NAT stuff. It's connected by ethernet to one 5th gen AE, which in turn is connected by ethernet to another 5th gen AW. Both AEs are in 'bridge mode' with manual IP addresses.
When both 5GHz and 2.4GHz networks shared the same SSID I had similar problems with devices not picking up the optimal connection when moving between AEs. Once I'd assigned another name to the 5GHz network on each AE this problem disappeared.
I can now move, nay skip, through the flat from AE to AE knowing that devices will stay on 'mojnet' or 'mojnet 5GHz' as appropriate.
I guess it helps that I have no 5GHz blind spots. On the downside, my flat would fit in your 4,500 sq feet house five times over...
Thanks for your reply. Yes read it right I am running the one 5g and one 6 g.
I actually turned off the Wi-Fi on the 5G. It is first device off modem. 6g is used as access point on first floor.
The 6G has the capability to broadcast two SSIDs, right? It can have a 5gz dedicated band I thought. Trying that it wasn't making the AC band stick to my 6+ or Air 2.
Though to be honest, the issues I have really mainly cropping up when I am in a corner somewhere a good distance from the router. Then it seems to persist as I walk into what should easily be AC range.