QuoteThe FAT TYPE partition is required and has been since the get go. Though the size requirements continue to grow. I go with 500Mb. You also have to flag it EFS and check the bootable flags as well...
That is correct; I always use cfdisk now to partition the disk prior to any install. There is an option in the "Type" to make the partition. Then you need to remember to formagt it FAT32.
Also, different distros require that partition to be mounted to a specific point. On some systems it's /boot/efi, on others it's /efi.

QuoteYour BIOS not allowing a Legacy boot environment is kind of surprising to me. Should be a spot to disable Secure UEFI, then enable CSM Boot.
Manufacturers started getting away from that a few yeas back; uEFI only now.
QuoteBoot Order: Ensure the Debian entry is not the only EFI boot option available. There might be a way to manually add or prioritize boot entries.
That is usually possible. There is typically an "add boot entry" option in the BIOS/uEFI settings that allow you add an entry - click on the drive that has the file, then scroll like a file manager to select the file. Once the entry is added, you can usually use the UP/Down arrows to prioritize the boot order.