Good news / bad news:
Bad news: There was a mistake in my very first post. I was wrong about my “known good USB memory stick”. I tried older and older versions of Armbian without success until I thought of double-checking the flash drive. It doesn’t work anywhere. That means my original report is not a reliable source of information. I apologize for wasting your time.
Good news: A kernel downgrade makes the system recognize and mount some of my non-broken USB3 devices. I believe these are the commands I used:
sudo apt install linux-image-current-rockchip64=21.05.9
sudo apt install linux-dtb-current-rockchip64=21.05.9
sudo apt install linux-u-boot-rock64-current=21.05.1
sudo apt-mark hold linux-image-current-rockchip64
sudo apt-mark hold linux-dtb-current-rockchip64
sudo apt-mark hold linux-u-boot-rock64-current
Now I have this:
$ uname -a
Linux rock64 5.10.63-rockchip64 #21.05.9 SMP PREEMPT Wed Sep 8 09:06:59 UTC 2021 aarch64 GNU/Linux
Unfortunately I am out of time for more testing today. (My downgrade goes beyond the one originally suggested, but now we know that my experiences were unusual because of a broken testing setup. I would like to try some bisection to identify exactly which edition of the kernel first breaks USB3.) But here at least is one data point showing a configuration that works correctly.
Thanks again for looking at my issue.