It's been a few days now and the network, including the iPhone, has been stable. The ONE exception was some issues with the entire 5 ghz WiFi network going down roughly 24 hours after the firmware was flashed. However, a reboot of the router got everything working and the issue hasn't been repeated.
So if you are updating to the tip of 20.01 on these devices and force your network on to non-DFS channels things seem to be pretty stable. Maybe it's worth a pre-emptive reboot after you install your packages and configure everything.
I still don't have an answer to the DFS regression, though. I may start bisecting with the mac80211 patch.