They don't pro-rate the included traffic quota for servers that don't run a full month?

No.

Yeah but if you're running something that is using a ton of bandwidth, presumably it's a service that needs to be online and reliable. Being cheap and constantly cycling out servers introduces complexity to your system. Hetzner could also easily change their terms to close this loophole if it's an actual problem.

You can automate it with zero downtime on kubernetes.

And since internal traffic is free, you just need to cycle the ingress servers.

Pay for the cheapest instance, get 20TB of egress. Churn em and burn em.

I bet that costs more than 20€ to do