Thanks Arvidjaar 
Well, as far as I understand your answear, there’s no way to get Windows boot manager in grub menu with Grub-BLS, and it might be less secure than Grub-EFI 
And there might be no way to prevent Grub2-BLS from putting itself in the first rank of EFI boot loaders.
Well, it makes me wonder why OpenSuse is promoting this evolution 
My security case is simple, my daughter is going to use her laptop in different places, some less secure than others, and I want her data to be protected in case the laptop is stolen. Clasical encryption scheme.
From what you said, I’ll give up with convenience, and get back to Grub2-EFI. Besides the good-looking GUI ( 
**) the main problem is keyboard layout, as the passphrase will have characters that aren’t on the same place on english and french layouts. But if there’s no way to start with french layout, I might just configure a passphrase with the “corrected” letters, she doesn’t need to know she’s typing “q” while she’s using the “a” key… 