The above method of creating users is apparently missing something. After swapping out the bookworm main server against a bullseye main server I have been able to log into student/teacher accounts from the same bookworm workstation. So bookworm workstations seem to be configured and working correctly at least.
Note to self: testing this required adding the workstation with gosa on the server, as well as running copy-host-keytab and manually removing Debian-Edu_rootCA.crt on the workstation with a reboot afterwards. -- Guido Berhoerster