I’ve done it!
I only had to disable secure boot. And I only needed to add a supervisor password to enable… disabling secure boot.
I’m on Linux now! 
But… wouldn’t you mind telling me how to add Windows to grub2-efi?
I’ve googled this:
https://forums.opensuse.org/showthread.php/482214-Manually-adding-a-new-entry-(e-g-Windows)-to-the-GRUB2-EFI-boot-loader-menu
…but it seems complicated. I’ve copied this to grub.cfg and I can see Windows option in boot menu but it points to nothing. How can I change “set root=’(hd0,gpt1)’” so that it points to Windows?
I have Win10 on /dev/sda4.