partitions 2 and 3 are kernel- and root filesystems of the 12.1 version, 4 and 5 of 12.2 version. So it is 4 read-only filesystems (squashfs, not ext4) where on normal vanilla ‘default’ Debian you have 1 partition root filesystem that this ext4 formatted and kernel is in /boot and its modules in /usr/lib/modules
If you really want things to get running again from the same SSD, what I do in such case is to pull out the SSD, plug it into some other computer and map its kernelblockdevice name ( e.g /dev/sdb) as main hd for a virtual machine. In that virtual machine you specify direct kernel-boot, so you need the kernel+initrd+dtb from the SSD, they are in partition 1. Pick the ones that represent haos12.2
As extra, specific the root=UUID=<the uuid of the filesystem>
In adition, add loglevel=8 for the kernel commandline.
Then start the VM and you should see all detailed bootlog, which might disappear again because HAOS might set it to level3 or so.
The same principle with real hardware, but usually x86_64 boards don’t have a serial port anymore so the kernel commandline option console=/dev/ttyS0 will dump into nirvana. Screenshots is not good enough sometimes, there are phases where a monitor might miss things, depends on various things. On ARM boards, there is usually some way to connect a serial console cable, so then you can do the same.
GRUB version2 is quite sophisticated, but if you manage/edit to delete all that A B stuff, you can point it to the proper kernel file and rootfs UUID.
What could be the case with Atom boards is that something is stored in UEFI flash. UEFI support multiple boot entries, they are a sort of device+file path of the .efi file to be loaded. If no boot entry is there, it will load <EFI-bootpartition>/EFI/BOOT/BOOTX64.EFI
Then there is secure-boot, something might have changed. Debian12 is 6.1.x kernel, not 6.6.x as HAOS uses, like RaspberryPiOS, it also uses 6.6 since a month or so.
Without detailed logs of GRUB and/or kernel, it is impossible to say what is wrong. I have a NanoPi-R6C which runs kernel 6.8 and has its own specific issues, but it has a dedicated USB-C connector for serial console that I can connect to a laptop or so, then easy to copy-paste bootlog/kernellog if needed.