Did another fresh install, this time the UEFI one.
So far, everything looks good.
I'm gonna check for two more days and if everything keeps smooth I'm gonna mark it as solved.
Nevertheless thanks...

For the USB flash drive I used option three.
As for the W10 install, the only thing I didn't do is to select unallocated space as the target.

Any comments on that?

Not that I'm aware of.
For that it would be needed for me to flash it, wouldn't it?

My flash drive has the win10 iso, downloaded from the MS website.
I've got UEFI, but there is no such option on the boot menu.
When I had just upgraded from W7 I'm sure I had this option. But since...

I understand your logic.
But that is not what is happening.
See.
91582
It simply won't go.

On the other hand, if I take this path... brings me to W10 from where I'm posting :-)
91580

Long story short: upgraded from W7 to W10.
Decided to have a clean start and made a wipe.
Always got the error 0xc0000225 when booting, except when the installation USB was plugged-in.
So decided...