Yes, thanks - that's a very good summary.
In my case, the first machine crashed whether it was plugged into peripherals or not. I have screengrabs for when it crashed and rebooted while it was just on my laptop.
I think there's no consensus as it's some kind of intermittent fault. If it was about external monitors - which it evidently isn't exclusively - then I'd expect the machine to crash as soon as the monitor was used. But I've had the machine working for hours, with no problem, then crash spontaneously despite nothing new being plugged in (or out). I'm wondering: could this be a GPU issue?
One weird thing: the second machine ran pretty well for a fortnight, before succumbing to the same problem as the first. The only different in that first fortnight is that I was in a workshop space and was plugging my mac into the mains only rarely. So that suggests some interplay with power management too. But all of this is grasping at straws! Sometimes when there's an intermittent hardware fault, it's easy to try and attribute it to the last thing that happened. And I've realised that's got me nowhere so far.
I do, honestly, think it's disgraceful that every time I speak to an apple engineer, I'm told this is a one-off, they've never heard of this issue before. This is now starting to stretch credulity. A simple admission there's a huge problem, they don't know what it is, it's not down to third-party issues and they need to fix it - that would go a long way to help.