I’ve heard of excess carbon buildup from running higher octane fuel than needed.
Yes, that will occur. Anecdotally, I used high-octane racing fuel/oil mix in a 500cc two-cycle motorcycle engine (check out videos of Outlaw Karts, they're a hoot).
Tore it down after a lack-luster season, and noticed it was thick with carbon. I stopped using the high-octane, went to premium, then regular. The thing ran best on regular and didn't build up any carbon.
Not mine, but this is a good example of an Outlaw Kart https://www.youtube.com/watch?v=pdIVyldojqw