For helping with meltdowns likely the dog won't need any specific training. They'll just need to be a calm-temperament dog with a strong bond with you. My Labrador cross only had the basic obediance training, and when I had a meltdown she'd come up and look concerned and nuzzle me, and it always made me feel better.
Now, if you want the dog to take you out of noisy situations or something like that, that'll need training. Maybe try figuring out a signal you could give the dog when you're overloaded. Take the dog with you to a noisy place, give the signal, and then lead the dog to a quiet place and reward the dog. After awhile, if the dog starts tugging on the leash to go to the quiet place, follow their lead and reward them when they get there. (I could probably benefit from an assistance dog who knows to lead me out when there's a fire alarm.)