I am by no means an expert, but here is a piece on how to clean up and remove wheat paste.
With paper over top of it of some kind you're probably going to need to do a lot of scraping and frankly, you're probably going to need to repaint it anyway. If you're renting I'd suggest talking to whomever you're renting from first.
If you're not terribly concerned about the cost and had some spare time and were handy with some basic tools, you could do something super over the top like buying a couple pieces of plywood and cutting them to match the wall and windows (with maybe an inch of leeway on the top to prevent damage) and do your piece on there. Bonus to that is that you can take it with you and possibly rejigger pieces of the wood to match new walls in a couple years.
I use PosterRazor instead, because it doesn't turn the image into dots. This means you'll probably need a high resolution image though, unless you're doing a small poster. I cut off the borders, use a minimal amount of tape to keep it together, and after the poster goes up you can't even tell it's on multiple sheets unless you get close up.
rasterbator!
love this app.
I am fortunate enough to have a silk screen kit, cheap one but it is large and i can do multi pieces if i have to.
I typically print cuz I am too broke for supplies but have access to a printer at work.
https://rasterbator.net works great for posterizing images, or you can poster print a design on a large posterboard at a local copy shop
Another super cheap and easy alternative I turn to often is to just spray paint. Draw a design on posterboard or cardboard and cut it out as a stencil then spray paint it onto newsprint or whatever paper you're using. Use a projector to shine an image on a paper then trace it with markers or whatever too.
Also, get creative. I've pasted target practice posters, newspapers, anarchist zines, eviction notices, etc. Just experiment. And get weird with it