"The mileage cheat" - cashback/bonus miles/credit card if you reliably process 3k per month via KK?

In https://www.reddit.com/r/Finanzen/comments/1tss7hd/businessclass_f%C3%BCr_lau_der_meilencheat_bl%C3%B6dsinn/ most answers are "only if you have a lot of expenses". What exactly does it look like when you have a lot of expenses? I don't know how I always manage it, but my Hanseatic debits are rarely less than €3,000, and I constantly add more when I'm on vacation. I think I would manage 30-50k a year, just refueling is a few hundred euros per month. I