Hi XXXXXXXXX,
The French Mastiff can be a bit aggressive and dominant so I might think about another breed. http://www.petplace.com/dogs/choosing-a-dogue-de-bordeaux/page1.aspx
You are correct, most of it is the person handling the dog. I too have owned giant and large breeds all of my life however the mastiff was not one of them. I have only had experience with testing Mastiffs for therapy dog work or for the canine good citizen. The French mastiff was not among them and those that are serious about therapy dog work need to be good handlers so that is who I come in contact with. You seem to know a lot about training and handling big dogs and that is on your side.I would say that when you see a breeder regarding a pup make sure to see all health guarantees on the parents and look at the parents temperament. That will tell you a lot about a pup. I would also suggest you give a puppy temperament test.
As for other breeds, the Lab , the Newfoundland, the bull mastiff,the Labradoodle, and the Leonberger are all laid back or ready to play dogs.
dog training consultant
24 years training & behavior,pet first aide, Therapy dog eval., PA Animal Response Team