I know this thread is a few months old, but I have an unpopular opinion.
Purpose bred crosses and mixes are fine.
*IF* bred responsibly.
This means getting required genetic health testing.
OfA and/or PennHip, at the minimum. Ideally hearts, eyes, and thyroids.
Proper temperament testing, and zero kennel blindness.
Ideally, your dog should be tested in some way, whether that being workability, or titles.
You say your dog is in protection training.
Do you know the different drives of your dog?
What do you know about genetics?
What is the female like?
How is your male doing with that the training?
Did you undergo a civil defense test prior to training? I'd love to know what your trainer's opinions are!
I myself have a breeding program that many people seem to like and support, and it requires crossing to select breeds, not all of which are mastiffs. It is not about variety.
There are a lot of "designer breeds" that are bred without purpose, and that I am not supportive of.
What breed is your dog?
Also, I'd like to mention because someone mentioned the American bandogge.
The term bandogge is not necessarily associated with a specific breed. Someone bred a dog line, and bastardized the term, in my opinion.
A bandogge is any tethered dog released onto man or beast, and excels at the task at hand.
Definitely not a dog for the average pet owner.
But, they necessarily have to be a mastiff cross, or even a cross or mix at all.
Haha sorry for the unsolicited wisdom.
If you have a handle on what I've mentioned, and you have a specific goal to breed towards that is a betterment; not a detriment, then I personally am for it.
If not, then I probably wouldn't.
But, it does sound like this is something you are wanting to do, so there's really not anyone who can stop you, as it is your dog, and at the end of the day, we all want the same thing: what is best for our dogs.