It's been several hours, but I have a new case (and a now-caseless older computer) and I was able to boot into the Qubes installer.
I have no fucking idea why Qubes on a M.2 drive can't be just plopped into a new computer and be seen as a bootabke device. It works fine with Qubes on a USB device...
Backing up and restoring is very easy with Qubes, so as long as you back up before migrating, it's not that big of a deal, but I'm still annoyed. Given how my day has been going so far, it doesn't take much.