I ran into the same issues so I created “serverless-plugin-local-env” which is an npm module that uses your credentials to get all the environment variables you reference either from your resources section or import from other cloudformation templates.
I also use “serverless-vs-code” to help debug my functions, as it allows you to setup your test input JSON pretty easily, and have multiple teest cases.
https://marketplace.visualstudio.com/items?itemName=MutualofEnumclaw.serverless-vs-code