Thanks @SwampDragons. Unfortunately, the / was not the problem. The original post was for 20.04.1 but I’ve also been trying with 18.04.5. I tried stuffing the url literally as you suggested:
" preseed/url=http://127.0.0.1:1234//preseed.cfg<wait>",
but it’s still not picked up (ubuntu 18.04.5).
Also (re)read the preseed guide. After reading the packer issue 9115 I’m wondering if I should be using a server iso other than this one:
ubuntu-18.04.5-live-server-amd64.iso
What does the “live” part imply?
Given the behavior I see (auto-fill for first few screens, then stops on the disk config screen), I’m guessing it’s using the subiquity installer instead of the debian one, which may mean I need a different iso?