EDIT: The crashing point had nothing to do with graphics/drm/i915. The last message displayed (for about a minute) would show graphics/drm/i915 but the issue would actually be immediately after that, at dracut initqueue hook. As far as I understand, it would somehow start to look for a memory stick that was mounted and included in fstab at the moment of making the image of the root partition with dd.
After remaking the image (and sending it to the server via NFS this time), I can see that issue is sorted (see picture)
The issue now, as seen in the next picture, is at switch root stage. Which I really don't know the meaning of but I'll do some research about.

The next picture is net/grub.cfg. I edited this file and got rid of the install and instsys commands because I thought they were irrelevant (as I am not trying to install an OS). I also tried this grub.cfg without the root and nfsroot commands which (according to my understanding) would use the file system in the hard drive, instead of the one in the server, and the whole set up seems to work.
It is only when it needs to get the root file system from the NFS server that I've got the error in the second picture. Apologies, the picture is blurry.

At this point in time, I imagine there is something wrong with the file system. I made the image with dd, moved it to the server, and mount it with mount -o loop /image /srv/tftpboot/openSUSE-XXX/rootfs. I read it's not the same to image a partition as to image a whole disk. Also, that btrfs is somehow special in regards to imaging. Is there a particular way to do this?
Apologies for the delay to update. Thanks again for your help and time to comment.