Yast should work if all OS use the same boot method and check for foreign OS box is checked . Yo can not mix MBR and EFI booting. As a general rule the boot can not see any OS that uses a different boot method.
Does anything you wrote have anything to do with $SUBJECT? If it does, it’s going right past or through me. I don’t want to see any selection that begins with “Advanced”, unless it’s the name of a kernel or a new memory tester. Any system that’s multiboot is advanced.
The issue is, YaST seems to be unable to eliminate the existence of more than one Grub screen/menu/page for containing whatever the (irrelevant) total number of stanzas comes out to be. This is determined by GRUB_DISABLE_SUBMENU=, which YaST seems unable to manipulate.