What i would really love to see in a developer email service is also some workflows baked into the emails. There's a ton of times you would want to wire up a service and just set it for transactional events but also as sequences and with a default set of templates wired up. Think Zapier meets Sendgrid for emails and a set of templates maybe wired for default dev flows like Signup, Verification, Billing receipts.

We do use SG for transactional emails and their email DX is frankly abysmal. Also not a big fan of html for transactional emails but sometimes you gotta do it.