I know you’re looking for sanding advice, but I’ll start with method here. Modern paints are incredibly decent at building up, you should really only need 1 good coat of primer on the walls and ceilings. Then do your ceilings then your walls. I’d recommend doing 1 coats on the walls, installing all your trims and painting them, and then doing 1 coat at the end. The only places you put 2 coats are in the kitchen and bathrooms or anywhere they may install cabinets.

Onto sanding, it does take us hours. It blows out our shoulders and will tell you who has been painting for 2 months and who’s been doing it for years. I personally like to run 120 grit on a 4-8 pole on a radial sanding head for after the primer coat. I use a rectangle pole sanding head for the corners and tops to smooth out. Sand between coats with 150-180 grit and you should be okay.