Backlogs is a cool idea! But I feel like it really wants to be a Trello Powerup. Show a popup asking for CC info after install, using Stripe Checkout. Then give people a CNAME record that displays a public board in an iFrame, available at backlog.yourservice.com. This would be like a 3-5 day project, instead of weeks, to duplicate most of Trello's functionality. And you'd benefit from the visibility in Trello's marketplace. You could really simplify your implementation and still validate the idea. Just a thought!