The regular purchases I started tracking are petrol, water (utility), electricity, milk, and phone data.

None of those purchases happen every single week. But we do buy those often enough and (except for petrol) in every place we spend time in over the course of a year. Plus they're similar enough in the different regions to meaningfully compare them.

On a weekly basis, we tend to buy meat and vegetables. They're so varied, it doesn't make sense track them. And we frequently buy from other shops offering different cuts / veggies, but not accepting btc yet.