I studied out well the three options, and I realized that the one that I need is bumblebeed. I don't care about performance limits of this solution.
Well I tried to install the bumblebee drivers both the latest (419) but also the 390. In both cases I get the pc stuck on Asus bootup logo screen and the fan of the gpu goes high. The system doesn't boot at all.
So I followed the guide for bumblebee you have attached in the previous post. The result is that I don't get anymore the gpu fun stuck at 100%, but I the laptop still stops booting when it is loading TLP (set the "text" parameter when booting).
But if I try to boot with
acpi=off option, the system boots up and I can log in without any problem (the only annoying thing is that I can't use touchpad and keyboard, they're not working in this mode). But I can't adjust brightness and
systemctl status bumblebeed returns that bb didn't find any valid switching mode, so the discrete GPU will remain always on.
I am really getting crazy, I don't know what to do. Thank you for the support, any help will be really appreciated.