🎉 Right, all good, it was the lack of memory.
So keep in mind if you’re going to update a server that has 512MB memory from Ubuntu 20.04 to 22.04 that 22.04 requires a minimum of 1GB to boot.
It would be a neat if, say, oh, I don’t know, do-release-upgrade actually checked how much memory your system has and told you that before carrying out the upgrade and throwing you into a kernel panic on first boot.