Yesterday I filled a few of the larger spaces. I wasn't sure if my early efforts at cutting concrete strips would yield anything useful, as the primary purpose was to get to grips with what the angle grinder does and how it operates. But a couple of the strips fit neatly into the larger gaps, so I got them supported and then cemented around them. The first one wasn't very pretty but the second one was nicer, and the third one nicer still. Not that it matters whether it's pretty, they are going to be concealed behind insulation panels and drywall by the time I'm done.
I think I got 18 gaps in the wall filled by the time I decided to call it a day. There are still a few more in that wall, including a couple I left because I couldn't reach them easily and decided to leave all the high ones to do them all at once when I get the ladder in there. There are plenty of big gaps right behind one of the studs, that may end up getting filled with builders foam because it's impossible to get much else in there. And now I've figured how to cement in a strip of concrete I've got a bunch more in another wall to fix before putting in the panels.
I think I confused the cashier at the hardware store, because I bought a 20lb bag of cement and one single concrete block. The block is going to get cut into strips. It's a nice new block so everything is even, and I can mark it with a Sharpie or similar so I know where I need to cut it. Then I can get busy mending walls again.