Ahem, I made the jump and updated Mountain Lion to High Sierra just to be able to keep the GTX 1070 RoG Strix installed.
Finally it's almost working after many failed attempts: I can login in my old account/OS and I also installed and have the nVidia Web Driver updated and loaded with the CUDA driver too!
I still am unable to boot from the SSD HF+ since I'm a noob with clover, but I use the UniBeast USB configured to load the HD and it's snappy and it seems to work well, although I still have to verify everything.
The GTX 1070 8GB is still seen as a generic nVidia card with 7 MB! I have tried to inject other definitions in the Clover Options but it did not work, and I do not see why.
If there is any suggestion, please feel free to enlighten me! TIA