Thanks again, but you are not going to believe this (yes - going to rub it in). I simply did what I had done before (your response above gave me the idea). In Magisk, I uninstalled and had it restore Images, then rebooted and updated again. It downloaded everything for .31, installed it, and rebooted - done!
Went to fastboot after that and reflashed the patched boot.img, rebooted, and all OK . I had the latest Magisn 20.4, but reinstalled it so it would still be there after reboots. As for the Magisk modules that someone said had to be uninstalled - no problems - they were there after I rebooted.
No MSM.
Again, you and NotATreoFan have been super helpful. If either of you go back to Samsung, and have some issues - I may be able to help, but I think OnePlus will be your home for some time to come.