That sounds like good work! One thing that I (think I’ve) noticed is that the rake task doesn’t actually use the same code as the actual sending does (like from the /admin/email test page). I’m pretty sure that I’ve had a case where it worked in the UX but not the rake task (or maybe it was the opposite?)
While it’s fresh on your mind, it you could see that at least when it actually sends it does so using the same code that Discourse does, that would be great.