Reporting back...Reporting back as promised... both nodes have been up and running for over 11 days, and not a single drop-back (from 160 to 80MHz) on the 5GHz-2 backhaul. With official ASUS-firmware, it would have happened within a couple of days. So far, no issues, no disconnects. I plan to do a full factory reset this weekend, and I'll report back how that went.
The factory reset went fine, and the node was automatically detected and added to the mesh (wireless backhaul). I re-configured as I had for the official ASUS-firmware, with 3 exceptions: I left the 5GHz-2
channel selection on "auto", the 5GHz-2 bandwidth on "20/40/80/160" and the backhaul preference on "auto" (though I only have a wireless backhaul). After a few days, the 5GHz-2 bandwidth dropped below 160GHz and the AiMesh connection quality showed "weak".
So I changed the 5GHz-2 channel selection to 108, the bandwidth to 160 and the preference to "5G WiFi first". It's now been running for over 9 days without dropping once, connection quality constantly shows "great".
In conclusion, the problem of the dropping bandwidth with certain settings is still present in this firmware, but these settings 'fix' it for me. I suspect it had to do with the auto channel selection, possibly having problems during a scan for radar.