Drywall tilted, uneven, poorly plastered - how to wallpaper?

Hello dear forum community, as you can read above in the headline, I have everything in my new room that one can imagine. Ok, now to my previous approach. So far, I have tried to remove the old wallpaper, but it does not work very well, various layers of paper remain. I will try to remove everything without residue. But what appeared was a very poorly puttied (poor gypsum used with cracks, holes, and high transitions) drywall that has a very old and rough plaster on the corners, which is very porous and crumbly. I would like to wallpaper this wall again with a normal wallpaper but first prepare all the substrate. Now the question to you. How exactly should I proceed and which products should I use? - Completely remove all wallpaper residues from the wall - Repair and smooth all holes or plaster everything with a thin layer of plaster? - Sand and apply a primer? The problem is also that the drywall panels each have a sloping stop and therefore do not form a straight wall.