Thanks for your comments @simos.
I had a quick look at distrobuilder a while ago but I couldn’t make it work with the official slackware.yaml you linked to - I will return to it sometime. Meanwhile the idea of migrating an existing VM seemed like lower hanging fruit - which now has only this “first boot” problem remaining.
Interestingly, I just saw almost exactly the same problem mentioned here (although the message title is misleading). Maybe an incus-migrate issue?