[Help] What breed should I get next? Looking for giants or large breeds only

I have a 2 year old saint right now, who I love dearly. He is great with people, not so great with other dogs. I think he is being protective maybe. He gets along with my SO'S parents dogs since he grew up with them, but most other dogs he seems like he tries to dominate them. Don't know if he is trying to play or attack them...don't want to let it get that far. Would an e collar be a good investment for training around other dogs?

Anyway, so I was wondering, should I get another saint? Otherwise I was wondering about a newfie or Berner. I want another giant breed. Don't know much about those breeds. Are either more calm natured than saints with other dogs or is my saint an anomaly?