Where do you live and what types of cars do you usually see on the road?
If it's rare to see a sports car or luxury vehicle, it's possible that premium fuel "sits around" in the fuel station tanks. As long as you're going to name brand stations that see high traffic, I wouldn't worry about getting the bottom-of-the-barrel crap.
Besides that, are you saying that you only use premium gas once a month?!?!
From the factory, these cars are de-tuned to prevent engine damage in the event that the user puts in a lower octane fuel. While the car can run on 87, it is highly discouraged and you must avoid high boost and rapid acceleration when using this lower grade fuel. These cars are built for 91 or higher. You can experience knock and engine damage from running anything less.
About 10 years ago, when these engines were a lot fresher, I ran some tests: 3 tanks on regular, 3 tanks on premium, tame commuting and rarely hitting boost. I got noticeably better fuel economy on premium, the engine ran a lot smoother, and at a 60 cent price difference between the two octanes, I got more miles per penny from premium, making it the most economical choice.
Side point: The factory Polestar tune isn't magically making the car faster. It's optimizing the car for 91+ octane, as Volvo intended, and unleashing the cars true potential.