Topping compound is your friend. My best ex wife was an artist with topping compound on uneven walls and ceilings. I still had LOTS of sanding to do, but she consistently got way closer to level than I ever could. Her finished standard was wall paper smooth so perfect wasn't quite good enough... Oh, yeah. I bought one of those seemingly bogus vacuum sanding attachments and it worked really good in the early stages. I still plugged up the shower drain with drywall dust...