Thanks JagoUK for all the help, looks like DFS events are not an issue in my location.
Here's my setup (stable over last couple days) for now.
Wireless AiMesh backhaul. QoS off, Smart Connect off, Roaming assistant off, Wifi Agile Multiband On in 5.1 and 5.2 bands.
Notes:
1.
Same SSD in all 3 bands. Router won't let me set same SSID in 5.2 so used following commands:
SSH to Main Router
nvram set wl2_ssid="MyHomeSSID"
nvram commit
Wait 30 seconds, reboot all nodes.
2. This open up entire 600 + 1200 + 4800 Mbps bands for my devices to share.
3.
My hope with same SSID is that if 5.1/5.2 go offline, devices will naturally switch to 2.4 (same ssid) and stay connected even at lower speeds. When the 5.1/5.2 come online (automatic or manual intervention), again devices will just go to that. Simpler setup for my client devices.
4.
Windows devices used the driver settings I mentioned earlier to Prefer 5 GHz band and Roam aggressively. They are connecting almost optimally t the right band/AP. Phones are connecting well, though at times not choosing optimal band/AP but still not bad.
5. Even though I am sharing the 5.2 band for AiMesh and Fronthaul, it is not slowing down things in practice. It is actually allowing my Wifi 6 devices to connect to main router at 2400 Mbps.
6.
I have a gigabit WAN connection and I have seen 900+ Mbps on my Wifi 6 device 20 ft away from the router. Yes, it happens.
Hope the setup stays consistently stable and doesn't fall like a house of cards
Comments/critique welcome. If these notes helps anyone, please use them and leave comments.
If any Asus engineer reads this, please first read JagoUK's review and take those suggestions, then add these:
1. Improve DFS switching implementation to not affect 5.1 and use it as a backhaul if you have to switch off 5.2. A normal 5.1 router which is not on DFS channel doesn't get affected by a DFS event, so should XT8 not when not on a DFS channel.
2. Allow 5.2 to be used as Fronthaul in the UI easily in addition to backhaul if the user wants to share that band. Sharing a 4800 Mbps channel with backhaul is still better than a dedicated fronthaul channel of 1200 in many situations.
Thanks.