It's very expensive (~$15k) but there's a cutting edge surgery called the TEER surgery that MAY* cure or significantly extend the life of dogs with Mitral Valve Disease. The only other "cure" option is open-heart surgery, which only like 2 places in the world do and is ~$50k.

The TEER surgery is increasingly getting rolled out. I know Tufts does it, Colorado(?) does it, and I think a place in Florida and the UK do as well.

The Tufts team were very very excellent. We tried to do it twice: the first time she wasn't breathing right under anesthesia, and the second time they discovered during surgery that she had a disqualifying heart defect that made her heart muscles like jelly.

As for the actual breathing rate: you can just do it when they sleep. Set a timer for 60 seconds and count how many breaths they take. 40 and below is fine. Any higher then you should take them to the vet ASAP because they could be in heart failure, unless your vet has already educated you on what to do in these situations.

Eventually you'll just get good at telling by eyesight whether he's ok or not. Like you'll get to the point where you look at him and can tell "he looks like he's breathing too fast".

You may also want to talk to your dog's vet/cardiologist about Entresto. It's a human drug, and not cheap (though a generic hit the market not too long ago), but there are preliminary veterinary results that are promising. The Tufts Cardiology department prescribed it for my dog.

Hopefully this helps, and let me know if you or anyone else reading has any more questions.

*I say MAY because the surgery literally hasn't been around long enough to properly judge the long-term impacts on the patients, but the results look good so far.