I brushed on first coat and rollered on the second.
Just don’t overload the brush,
I think I used a 4” brush, but I have a much smaller area to cover than yourself.
It may also be stating the obvious but wear goggles and gloves.
And, presuming you’re lying on your back, lay to the side of where you’re painting above, so don’t paint above yourself.
If that makes sense.
The fumes off my paint were horrible, so paint up wind if possible and reposition yourself under the unpainted section.
I don’t think a wallpaper brush would be ideal, the proportion wouldn’t be right.
Wouldn’t hold enough paint properly.
But 🤷♀️try it and see.
Or perhaps do as Mike above suggests.