the only bully dog I think is really mellow almost all the time (ie every single day of every single one of their lives...) is the American Bulldog. Always really laid back - from what I’ve seen.
There seem to be more and more lab attacks, maybe its overbreeding.
Mellow is not the question. “English” Bulldogs are completely unlike their ancestors. Most important issues: not likely to snap, and not likely to hold on. Labradors are so hugely popular right now, they are likely to produce a few unstable nuts. Percentage is what counts.