Thanks for your point of view.
I tried to install the NVMe SSD, but failed, as it requires an NVMe driver which can only be installed when the SSD is connected internally.
And as I didn't manage to boot the computer from my Windows start medium USB stick, and it will not start from the NVMe...
I have seen which explains (between 1:00 and 2:30) that PCI Express Version 3.0 and Maximum Link Width 2x are minimum, and 4x is highly recommended.
Unfortunately, my Toshiba Portégé Z30-C 12Z only has Maximum Link Width 1x.
Do you agree that I should better get myself an Samsung 860 Evo M.2...