I got my nvidia working with external monitor. It was weird, apparently you need to set GRUB_CMDLINE_LINUX_DEFAULT and add nvidia-drm.modeset=1

GRUB_CMDLINE_LINUX_DEFAULT="quiet nvidia-drm.modeset=1"

and
grub-update

It had a persistenced issue every reboot too, the fix:

/etc/init.d/nvidia-persistenced stop || true
pkill -xf nvidia-persistenced || true
dpkg --configure -a
apt -f install

Then to see what's what:
dpkg -l | grep nvidia-persistenced
nvidia-smi | grep Persistence-M

It should be enabled and will now save the configuration.

I'm not sure if this is an nvidia driver issue on Devuan or just something funky with the Stellaris - it has an AMD 7840HS with AMD 780 and RTX 4070 and I'm plugging the monitor in via mini-display port. Either way, I'm posting it for whoever might need it in the future, including me .