You’re assuming that you “must” poll at the absolute API rate limit and arriving at a 750 / 15 = 50 equation.
The RESTful Direct Message endpoints are generally intended as backfill for Direct Message events potentially missed from the Account Activity webhooks - for example, if your webhook is unavailable and secondary attempts to redeliver are unsuccessful. That said, they can be used in the manner you suggest, but the use case is likely to differ. You’re free to choose the best polling interval for your application.
Twitter’s developer API platform has aged, and is in the process of being refreshed with new functionality on a new, more reliable foundation. As part of this, the new Direct Message functionality represents significantly enhanced features (JSON-formatted message objects with additional interactivity) as well as much greater access to message history. Some trade-offs have been made in order to assure stability and scalability.
For real-time access for Direct Messages for larger numbers of users, the Account Activity webhooks are what you should consider.