Any tips for designing a cloud application testing strategy that ensures reliability and scalability without causing bandwidth problems?

I would highly recommend you shift from "testing" into "observing".
Look at Dave Farley and Mike Bryzek's youtube on "Testing In Production"

https://www.youtube.com/watch?v=Nmu4URA7pSM
https://www.youtube.com/watch?v=z-ATZTUgaAo

"Testing in Production" is not waiting until production to test, but it IS about ensuring you have end-to-end monitoring in place.

You can then shift to Adapting to load to ensure reliability and scale.

e.g. If you're worried about <2s response times for your API or App webserver, monitor average response times.  If you also have your components under monitor (CPU, Memory, etc.. for your web, database, and other backend elements) you can see what causes response times to increase with load.

Now you can forecast autoscaling strategies, and scale up where needed, when needed.

Hello, testing for the evaluation of which KPI?