This is my answer to my unfortunately already closed thread:
I did like I was advised in the original thread and in Ermanno’s great tutorial and everything went fine without the slightest glitch.
In short:
I didn’t touch the Win10 drive and created 3 partitions on my 2nd SSD (efi+root+home), let the installation finish, rebooted and there is my GRUB menu as desired.
Thank you everyone for your efforts and your patience!