A well bred American Akita would fit the bill also. I know some can be same sex aggressive but, that has never been the case with the ones I have had. Both were females and got along fine with my Sibes. Super smart, problem solvers for sure, family dogs but, protective and will keep other people away.
I think all the suggestions you have been given are great!
I have looked at other breeds a few times. When I originally wanted a breed of my own to show/breed we had Alaskan Malamutes as well as Siberian Huskies. I wanted something that was more "challenging" than my Mals. It took me abut a year to decided on breed then another year to find a breeder! LOL. I looked at just about every CKC breed at the time. I even picked my "retirement" breed back then (Italian greyhound)