You can train ANY psychiatric service dog to body block people from approaching you. They can learn tasks such as lying down directly behind you in a line in a store to prevent people from standing up on top of you, or circling you to keep others away, or moving between you and someone else on command. This will go better with a larger dog, but even a medium sized lab or golden is perfectly capable of doing this work.
I don't see many berner service dogs for the reasons mentioned (health, size, and age), and while they're EXTREMELY trainable and could easily do service work for someone, say, in a wheel chair, most are incredibly emotionally attached to their owners, and having an owner in psychiatric distress is going to cause most berners a whole lot of anxiety, preventing them from working very well. The right breeder and the right dog might exist, but I would absolutely not narrow down your search to only berners because of their size and looks.