Ciao Pietro,
I just tried an installation on one of our Mac nodes by following the simple steps I outlined here. I guess this should work for you as well, let me know.
Regarding the warning you saw when installing with brew, most probably they come from the conflicting installation in your system. In general, you should avoid having different versions of the same software in the same environment. Something like a Python virtual environment like the one you can create with the instructions I linked above kills two birds with one stone.
Alternatively, if you want to keep the brew installation, make sure you always have exactly one version of ROOT installed on the system (although I would advise against this in general).
Cheers,
Vincenzo