So I just completed the backports installations and ran into the same keyboard issue. In my case, I was able to start squeekboard from an SSH terminal as others suggested. Also, if I run sudo update-alternatives --config Phosh-OSK and select stevia as the default manual keyboard method, that functions as a workaround that will survive reboots.
Also for whatever reason, some type of accessibility screenreader is active after installing backports and I have yet to be able to figure out how to turn it off.
I also ran into slews of codeberg too many connections errors, but just kept chipping away with sudo apt update --fix-missing again and again until I got all the packages downloaded.
Then I finished the remaining package installations with sudo apt dist-upgrade, and ran sudo apt autoremove -y to clean-up.
After the 1st reboot, I got the welcome app, which was unusable at default 200% scale. Will update this thread with some screenshots when I can.
The bug where switching to 175% scale shows as 100% in the dropdown menu is still there, as reported here
I headed off the LUKS issue by first installing the unl0kr pkg from the codeberg backports repo as advised, editing /etc/crypttab as suggested, and regenerating initrd.img. Rebooted and tested unl0kr first, and then proceeded with installing the bulk of the backports packages.
Excited to have so many up to date versions of apps. Will be testing them throughout the coming weeks and updating this thread with any notable observations.
Thank you @galilley for your work on this.