Something similar, and perhaps even simpler: Make a few SNS topics that lambdas can subscribe to - either one per method or one per method type, and then filter more in code - and push notifications to those from your service layer as needed to invoke async workflows. I've been doing this recently and it's pretty pain-free.

Been meaning to look at Step Functions for a lot of this async work stuff, but haven't had time yet (plus it's nice to let these things mature a bit before diving in).