It seems like this is a bit more difficult on MX4 (no space left in the respective filesystem) - but with a bit of persuading I was able to get the update installed following these steps:
sudo ubports-qa install xenial_-_edge_-_wayland(finished with error)
apt -o Dir::Cache::Archives=/home/phablet/Downloads full-upgrade(processing the updates using Downloads folder to save the packages to prior installing)
With the above modifications I can confirm the screen being black on startup :).
Unfortunately though I am still getting
what(): SDL_Init failed: No available video device. And this could be an issue with my Wayland modifications in the code - which now seems more probable.