Kenesis and SES, maybe. But Dynamo would be pretty easy to replace with something like Mongo provided that you abstract your databases access code and don't have a lot of backend specific code missed with your business logic.

A lot of serverless apps use Graphql for this purpose since Graphql is backend agnostic.

I would not be not easy to implement the full generic functionality of Lambda and API Gateway. But I think it would be fairly easy to just write some application-specific Express code to handle the API routing, and add some wrappers for Lambda calls, in order to migrate a single application.