RE setup and testing: Trust (as is most devops one-time setups). Once the initial email setup is complete, you typically aren’t paying with it much. The black swan outages aren’t really an active concern.
RE PII: email is non-secure and shouldn’t have sensitive data in production either. Also, dev/test shouldn’t have PII in regulated industries as a good hygiene practice (I’ve worked in healthcare, finance, and national security contexts).
Re licensing: I appreciate your openness and clarity on the licensing of the gateway engine as AGPL vs MIT for the rest. There’s a more modern licensing approach with FSL-1.1-MIT. It may be a better fit for customers (ie clear licensing terms when using a paid license and less concerns if the business goes defunct or pivots) and for your business plans.