It takes a couple of years to train a service dog. So, I wouldn't actually choose a large breed, as by the time they are fully trained, they would almost be ready to retire, since they need to stop working once they have age-related problems such as failing hearing or sight, joint problems, etc.
That would mean being back to training a new dog in just a couple of years. Dogs which you are never sure won't wash out after months to years of training to begin with, for any number of reasons.
Rescuing dogs as they come is another thing altogether, of course.
I can't talk, grooming-wise, having a standard poodle. And, Dh said that he wouldn't rule out another poodle one day, so...! He really didn't know the breed before Tango, and was pleasantly surprised.
Also, I now have the issue of having any breeds who would not mix well with livestock, from chickens to cows. All of my neighbors have some form of livestock, and you need to be able to take your dog out with you without a problem, and have your dog not chase anything if he/she gets out, as people will shoot a dog that goes near their sheep/goats/horses/cows/chickens etc. etc. etc.
What about insurance? Didn't you discover that your insurance wouldn't cover chows, and various other breeds? This was back when you had Blackie, I think? I imagine that would be a consideration, too.
I hadn't even thought of a golden retriever's energy level.
That's a fun way to interact with nice breeds that you don't have...visit someone else's! Irish wolfhounds are very nice dogs.
Love All Life, Thank You For Posting! :)