Just a thought, I admire your endeavours, but it may be easier to get a plasterer to do that - if you have a look round locally and see if any work is being done, perhaps a helpful plasterer could pop round with a bucket of plaster and fill that for you.
As I said, just a thought.
Going for it. So far so good… easifill maybe tomorrow evening on a smaller area before moving into the hallway and the more visible spots.
Getting a paddle mixer for the drill was a bit of a revelation - just been using a small scraper til now.