I'm glad the situation was resolved to your satisfaction. From what you've said, after you booked your cruise the rules changed over requiring a booster to board the ship. Your doctor advised not getting a booster for health reasons. Regent would have to deny you boarding through no fault of your own. Since the rules changed after you booked, I feel Regent owed you a refund without question. There was no reason to put you through such mental anguish a month before your cruise.
Having said that, I wouldn't be so sure a booster won't be required for your December cruise. We got Covid in March and were boosted in May. Our doctors said wait two months. Many say wait 1-3 months. Your doctor said don't get one at all. You might consider going back to him and reconsidering whether that is the best decision.