What do you have in your grub cfg now, and how are you going to re-add it to the config during system updates? If the grub.cfg gets regenerated for each new kernel that gets installed, any manual changes to the cfg will get wiped out.
#Boot { GRUB, ISO, IMG, DD, ... } LEGACY | EFI | UEFI ...
mild salmon
mild salmon
I haven't done this multi boot iso thing in a while, except using ventoy, but that doesn't solve your problem. This is imo the definitive documentation for this, see if it gives you any ideas: https://wiki.archlinux.org/title/Multiboot_USB_drive#Manual_configuration
Here's the specific sniper how they do the loopback with partition uuid, instead of (hdx,y) style:
insmod search_fs_uuid
search --no-floppy --set=isopart --fs-uuid 123-456
# later use inside each menuentry instead
loopback loop ($isopart)$iso_path