I was just researching why it is that the 6.5.7 rpi 64-bit image does not work by default with the official Raspberry Pi 7" touchscreen. It should work IMO: it's a very likely piece of test hardware. I found

https://forums.raspberrypi.com/viewtopic.php?t=358424

which I guess is related to why the config.txt settings are changing too. I was not able to get 800x480 mode with hdmi_group=1 and hdmi_mode=3. On Raspberry Pi OS, as explained in the link, kmsprint and kmstest are available. But on b2qt, tvservice is still there, and gives this output:

root@b2qt-raspberrypi4-64:~# tvservice --help
tvservice is not supported when using the vc4-kms-v3d driver.
Similar features are available with standard linux tools

So it seems we should include the newer tools in our images.