The over 80's I know have all had 2 doses but regardless as you say there is good evidence a single dose is effective.
data here
* parents had both doses, no real reaction (same with inlaws). arm soreness and very mild headache were the worst for all four inlaws and parents (mid 70s age)
* wife had both shots (health care). pfizer. Reaction to the second was 1 1/2 days of extreme fatigue and then gone the same as it came - all at once
pfizer has since been found to be about the same effectiveness 3 weeks after first shot as it is after 2 (this is short term, will long term be different? don't know).
As a low-end tub (205 pounds, but just over the line for "obese" bmi) I had the first pfizer shot so far - mild arm soreness, maybe a headache (I always have headaches, more often than not, so probably not the shot, but if it was the shot, we'll call it "cute" compared to someone who gets real headaches).
Second shot in two weeks. Not dreading or looking forward to it, don't really care.
Second data bit - inlaws (my generation, not parents) all got moderna shot but got covid about 1 week after. They didn't have protection yet and everyone in their house got it. Nobody was seriously sick, but with their age and physical condition, money bet would be the shot didn't do much at that point (fatigue for a week or so for one, fatigue for the other for a little longer, then gone. All three young kids got fevers and were tired for a couple of days).
If they were exposed several weeks later, they probably wouldn't have noticed. BIL went back to work with an IT group in an enclosed space, one dude spread the droplets a day or two prior to BIL's symptoms and that was that. His lack of care wasn't due to having had the vaccine - he hasn't paid much attention to covid the whole time, only his wife has.