Projects like this seem like a great idea, but deliverability seems like a big concern that is hard to measure unless you have a reasonable amount of experience. What are best practices for using/selecting an ESP if you were to use a project like this and want to ensure reasonable deliverability?
Deliverability is a non-issue if you hook tools like this and Sendy up to Amazon SES with DKIM and verified MAIL FROM settings. I've been doing it for years and never once had a deliverability issue. I've also sent directly from EC2 instances without issues but SES is so cheap and reliable I've stuck with it.
Also, the quality and size of the starting list matters too because a large list (like the OPs) or unengaged list can have a high bounce and low engagement rate, which factors into future deliverability.