That absolutely mirrors my experience - I never spoke to anyone in Sicily or Venice who didn't immediately switch to non-dialect Italian as soon as they realise

That absolutely mirrors my experience - I never spoke to anyone in Sicily or Venice who didn't immediately switch to non-dialect Italian as soon as they realised I was a foreigner (fairly obvious from my fair hair and blue eyes!) who could hopefully just about understand Italian but plainly couldn't understand their dialect. I did once watch a play entirely in Sicilian; it was very beautiful and mostly incomprehensible to me!