Oh, it seems to have progressed now! I reinstalled Ventoy onto my USB by disabling the Secure Boot option (don't know if this did it, since I disabled Secure Boot in the BIOS, but maybe it did) then recopied over the iso. After running it with Expert Install, it gave the same "no installation media" error, but then, after I ran "sh /lib/debian-installer-startup.d/S21ventoy-loop", it finally detected it.

I don't know if this was because of what I did to Ventoy/recopy, or if it would have worked before with just the command, and I just gave up after it froze too soon, but it finally worked. I'll continue with the installation and see if everything works. I'll report back if I reach another error or if it finishes with no more problems.

Edit: it then froze when clicked Network Config option. It seems like the freezing was a separate issue from the media detection issue (or maybe it's just that error = freeze at times or something like that)

Edit 2: Yeah, it probably is a separate problem. Doing it again, I easily got past detecting network. Maybe the freezing is just a hardware issue, or maybe an idling issue?

There is nothing in /lib/debian-installer-startup.d/S21ventoy-loop. The folder itself exists, though. Edit: Sorry, my bad, thought it was a folder. The file contents are:

sleep 1
DEV=$(blkid -L Ventoy)
mount -t exfat $DEV /mnt
modprobe loop
losesetup -f /mnt/devuan_excalibur_6.1.1_amd64_netinstall.iso

Is it possible that the problem is that the boot order in my BIOS doesn't start from USB? I doubt it, but I'm not too familiar with it to know either way.

Yes, the usb with Ventoy is formatted with exfat and is labeled "Ventoy"

I disabled secure boot and two windows-listed settings in the BIOS, and it now actually continued, but it continued to a "Detect and mount installation media" window:

"Incorrect installation media detected"
"The detected media cannot be used for installation."

"Please provide suitable media to continue with the installation."

Then it went to:
"Installation step failed"
"An installation step failed. You can try to run the failing item again from he menu, or skip it and choose something else. The failing step is detect and mount installation media."

I tested on a virtual machine on a Windows 11 laptop, and everything seems fine there (just quickly went through the install process without really caring about the settings). So I assume it must be a hardware issue, I guess. I'm installing on a newly built PC: AMD CPU and graphics card with an MSI motherboard, and the iso file is on a usb stick. Could that be the problem?