I had always heard professional dog training was only really worth it if the dog was still a puppy
Nah.
Learning for any living thing is easier when young, but just like people dogs continue learning their whole lives. For some things it’s even easier because they understand more.
You said your dog “gets tired” and stops reacting to someone going by?
Have the person keep going by, everytime your dog doesn’t give a shit, shower him with praise and give him a treat.
He’ll understand that’s what you want, and he’ll start doing it everytime.
Positive reinforcement always works better than negative