You don't have to worry about cache rules, but what about your bill with Cloudflare?

5$/month plan is kinda open ended, you pay for each request above included in the plan (0.5$ for 1 million requests)

With a high-perfomance language like Golang for example, I could write a script that's making 500req/second to your site. That would cost you 20$/day.

With Github Pages it wouldn't cost you anything (or they might disable your page if it costs too much bandwidth, not sure)