So, after my last message, I ran across a general mdadm setup guide at https://raid.wiki.kernel.org/index.php/Setting_up_a_(new)_system and, in the "booting" section of that document, it said to add GRUB_CMDLINE_LINUX="domdadm" to /etc/default/grub. Didn't have that, so I added it, did the checks mentioned in the document, and reinstalled grub to both EFI partitions.
Still didn't work. Then I found https://linoxide.com/linux-how-to/grub-rescue-commands/ and was able to start the installed kernel with the command sequence: set root=(md/1) linux (md/1)/vmlinuz root=/dev/md1 initrd (md/1)/boot/initrd.img-4.19.0-14-amd64 insmod normal boot This still ultimately died with an error saying the EFI partition had an "Invalid FSINFO signature". Searches on that phrase mostly seemed to talk about secure boot problems, so I double-checked the bios options and secure boot was off, so that wasn't it. So I went back to the linoxide.com document and went through all the steps that I hadn't already done manually: Running os-prober and update-grub. Then grub-install to the EFI partitions again, reboot, and... success! Based on this, I'm guessing that the original problem was that the installer forgot to include mdadm support in its grub options, even though it was configured with an mdadm boot device. And then I missed a couple steps after adding mdadm support, so it didn't all get installed to the EFI partitions correctly. -- Dave Sherohman
