Haven't tried that, but something else already helped: While it was idling with fsck in tty1, I went to tty2 and entered: apt --fix-broken install ... and it did/resumed full upgrade. (Interestingly, this time it did not complain about no space in / and /usr.) When it finished, I tested startx and it brought GUI. Not sure now but I think that I then rebooted and it went it into GUI as expected. So far - so good. Few red [FAILED] warnings during CLI phase related to not starting UFV, Shorewall, and minissdpd services, so I need to check for that.
A subsequent apt --fix-broken install (or some other command) only complained about some initrd issue with kernel image 4.19.0-20-686 so I removed that image and stayed with 4.9.0-19-686.
After that, apt autoremove freed some 500MB of old stretch packages so now / is about 97% used, while /usr is still 100% used.