Hm, unfortunately nouveau.debug=debug did not improve things enough to get a
log from the beginning. But after reading through
https://freedesktop.org/wiki/Software/systemd/Debugging/ and configuring it
journald with:

#ForwardToWall=no
#MaxFileSec=1h
#MaxRetentionSec=1month
#RateLimitIntervalSec=0
#RateLimitBurst=0
#Storage=persistent
#SyncIntervalSec=20
#SystemMaxUse=1G
#RuntimeMaxUse=1G

I managed to get what I think is a full log from the beginning with
nouveau.debug=trace with hopefully all the information needed. There's other
stuff in it, but if you filter the "kernel" lines only this should be same
output as dmesg.

Please have a look at it. If this log is still not helpful I hope I will manage
to get this netconsole thingy running...