Just removed wallpaper that's been on for several years.

try renting a hopper and spray your wall with dry wall mud thinned down once the wall has set a while you can smooth it out dry then paint

I've applied joint compound in a thin layer, then lightly sanded to blend in with wall prior to painting

I would probably give up trying to fix the walls back to pre-wallpaper finish after the coat of drywall compound to smooth the walls didn't work. You could put up new drywall but that's an expensive option. You could try wall liner, that is often used on old cracked plaster and damaged drywall surfaces to provide a smooth (or textured) surface for painting. It comes in many different finishes and is paintable. I would hire somebody to put it up for me, as I am no good at doing seams that don't overlap!

I did the same thing years ago. I turned to my mom, who had been hanging wallpaper for years. She advised using warm vinegar water with baking soda. A bucket full of hot water, 3 cups of white vinegar, 2 cups of soda and a sponge ( a clean sponge mop works great). Apply generously and let it soak in. The paper should peel off and the vinegar soda water will remove the sizing easily. I removed the paper off one wall in less than 2 hours, after struggling for an hr. trying to get it off with a scraper on a 1’ square section. Hope this helps.

Just redid both of my bathrooms with the same wallpaper problems. What a mess! But, let me tell you that Brocade paint on the walls is the answer. If you are not familiar with that, it looks like "stucco". I had a painter come in and brocade both of mine and he only charged $75 for both. I had to paint it myself, of course, but it is gorgeous. (I think I spelled brocade right)