Possibly related to:

https://issues.guix.gnu.org/35296 ‘gdm doesn't start at boot’

The message in Xorg.1.log is similar.

It seems suspicious to me that 'gdm-shepherd-service' doesn't have 'elogind'
in its requirements, but 'sddm-shepherd-service' does.  I tried adding
'elogind' but that didn't fix anything.

There's something else that seems suspicious to me: elogind can be started
by two methods: by 'elogind-shepherd-service' and via D-Bus.  It appears that,
if elogind is started via D-Bus, then it can't be started via elogind-shepherd-service
anymore, so herd can think elogind failed to start even though it was started with
D-Bus?  That didn't seem the case on the test VM though ...

Greetings,
Maxime.