Dear Pascal,
If Windows boots in EFI mode :
Mount the EFI partition on /boot/efi.
Install grub-efi-amd64.
Boot some Linux media in EFI mode.
Chroot into the Debian system, mount the usual pseudo-filesystems
(/proc, /dev...) and the EFI partition.
Run grub-install.
Run update-grub.
Done.
Your simplified solution nailed it! Thank you.
I mark this thread as solved basically because of this small paragraph. So if you are reading this in the near future trying to find a solution, this step-by-step and some duckduckgo will get you into business.
There are more details for a complete response, and some commands needs to be in a different order, that I'll reply later in this thread, just to make sure the procedure that I made and worked flawlessly, is registered for posterity.
For now, if you are in a hurry, this answer above will get you in the right path.
My best,
Beco