Hi Dillthedog,
thank you for your quick answer.
I’m not able to provide the -A logs, because that won’t finish even after 30 min 
I guess its waiting on systemd…
Moreover, at that state of the system, I would not be able to install Sam’s patched kernel, because apt-get suffers the same problem.
However your links pointed me to the right direction, according to
fzinken at Headless Pi boot failure - #3 by fzinken
I added disable_fw_kms_setup=0 to config.txt, and all of a sudden, sshd lets me in again without --noprofile --norc -i
At first glance it seems that systemd now works properly again.
Is this setting save to use though?
Btw, the update clobbered my config.txt settings, I’m pretty sure that I once added some hdmi settings already, and my mpeg2 key is also gone. I think that happened in the august update and not in the previous one to buster, I did earlier that year.
Was that expected behaviour?
Thank you very much for your pointers for now 
Best
Martin