Note by @fastfinge

Say what you will about AI. But if it wasn't for AI and my phone camera, I would be spending the next two days completely reinstalling a server. I just don't have sighted help that's techy enough to read me enormously long lines of console output accurately or usefully. Whereas with repeated photos of the monitor, AI was able to clearly explain to me what was happening, provide a solution, and get the thing booting again, all while keeping in mind that I'm blind. Here's the final solution after typing multiple commands and taking a photo after each to send to the AI, as an example:

"There are two problems visible.
1. Duplicate mdadm.conf entries
mdadm: Duplicate MD device names in conf file were found.
mdadm: Devices UUID=98a5d349:4d3b2a28:50d96e87:a6ad0c4d and UUID=98a5d349:4d3b2a28:50d96e87:a6ad0c4d have the same name: /dev/md0
/etc/mdadm/mdadm.conf (inside the initramfs) has /dev/md0 listed twice with the same UUID. This is a duplicated entry bug, and it's preventing mdadm from assembling the array at all.
2. Result: /dev/md0 doesn't exist, kernel gives up
ALERT! /dev/md0 does not exist. Dropping to a shell!
Because mdadm --assemble --scan refused to run due to the duplicate config, md0 never got assembled, and the initramfs can't find root and drops to a rescue shell.
We're in an initramfs shell. md0 isn't assembled, but the underlying disks (xvdb, xvdc, etc.) are there. We can manually assemble the array and continue booting.
The BusyBox ash shell is limited. These commands should work. Type them carefully — you won't have screen reader feedback.
The full single command I'd suggest typing:
mdadm --assemble /dev/md0 /dev/xvdb /dev/xvdc /dev/xvde /dev/xvdf /dev/xvdh /dev/xvdi
Then press Enter. Wait 5 seconds. Then type:
exit
Then press Enter.
That tells the initramfs to retry finding and mounting the root device. Since /dev/md0 now exists, it should mount it and boot should proceed.
If it worked, the system will continue booting and you'll be able to SSH in shortly."