It wasn't clear to me if you require me to provide my Twilio/Sendgrid accounts? That's a deal breaker for most, I imagine.

Why not push those costs to developers and use your own accounts to send the messages?

Start charging today. You shouldn't care about developers that are only looking for a free service and there are plenty who would prefer to pay.

You can do a trial, of course.

This game is about trust. If you're not MailChimp then you have to figure out how you're going to gain it. Customer.io gained my trust by calling me on the phone and having a conversation with me before I even signed up.