My company uses separate stages (through serverless framework) for each branch of feature development. CI uses these to test, running within AWS infrastructure. It works well for isolating test environments. We lean more on unit testing to debug the service with external dependencies mocked. We try to avoid situations where we are adding log statements one-by-one; CloudFormation deployments take several minutes, so iterating small changes is indeed frustrating. We thought about localstack, but haven't tried it. I'm worried that it will increase our IaC complexity; everything will have to deploy to both AWS and also localstack.