tl;dr: the 7968 build doesn't work for AX clients, even if you start from scratch.
Had some spare time over the holiday and giving this firmware release another shot.
I did what some of the others suggested, and completely reset the three nodes (main AX88U, node AX88U, node AC86U). I rebuilt all the settings from scratch (did not restore previous settings).
Unfortunately, this suggestion changed absolutely nothing vs. the "dirty upgrade" (simply loading the new firmware without rebuilding settings). I'm afraid that suggestion to build from scratch is more placebo than practical.
After a half day of use, I observed my AX client (iPhone 11) slowed down to sub 2Mb/s (first attachment).
I disabled 160 MHz, then reconnected. It worked for a while, then slowed again on AX. I then disabled AX on 2.4Ghz as I don’t have any need on that band. After initially seeing good results, after an hour or so, speeds dropped again on my AX client to being very slow. I have since disabled AX on 5Ghz and now the AX client is connecting via AC with no slow down (second attachment).
So what is the point of this router?? I can’t use its trademark feature of AX connectivity with this latest 7968 build. Have to revert to the 6436 build, which doesn't have the Wifi Agile Multiband capability nor the certified Wifi 6 goodness.
One interesting observation: when using Wi-Fi analyzer, I noticed that the speed on 5Ghz was stuck at 40MHz channel bandwidth even through the setting was “20/40/80.”. Changing the control channel to a different channel and restarting finally set the router and nodes to 80MHz.