I'm not an expert, but it doesn't sound like normal aging to me. I've had dogs aged 11, 12, and 13, and they didn't display any of the extreme reactions you're describing. Plus, Rainy's just barely into official "senior" status -- relatively young. I would talk to your vet again, as you're planning. Getting a full thyroid panel sounds like a good idea.
As I understand it, she became spooked at fireworks before you moved into your present house, but it's been worse since then? There have been a couple of GT hounds who developed extreme nervous reactions to noises, such as a fire alarm with a run-down battery. Humans couldn't hear it, but the hounds sure could. Outside construction noise that didn't bother the humans was another nerve-wracking experience for a hound. The dogs became generally fearful and nervous. Maybe do a whole-house check for weird stuff like that?
I'm sorry you and Rainy are having to go through this.