I did quite a bit of research on that just recently for my startup and here is what we landed on and a super happy with:

Pipe all event tracking through Segment.io including the triggers for transactional emails

Setup Customer.io to listen to the transactional event triggers from Segment.io

Create workflows in customer.io to send follow up, churn and other non transactional emails based on other events coming from segment.io

This makes it super easy for non engineers to change the setup

Customer.io provides email templating, analytics, AB testing and everything else you might need