Or, demo the interior board behind the wall paper, then put up what you want. Preferably drywall. It sounds like a lot of square footage you are talking about. And, drywall is cheap. Plus that’s be the quickest.Mud the whole wall, and sand it smooth. I hate wall paper.
Do this. It works. I didn't do it personally but my company did on a reno we ware doing. Add 3 drops of dish soap to the mixture and hit it with a putty knife after a few minutes.Vinegar and water, we did a bunch that was on lath and plaster. Small squirt bottle with a 50/50 mix of vinegar and water. Spray till generally wet, let sit a minute or two, then scrub with a brush or hit with a scraper. . . I've done it on drywall as well but it can be a PITA.
As others have said, a steamer is best.