Niko Neugebauer makes a few recommendations:
Optimizing costs in SQL Managed Instance isn’t a one-size-fits-all situation—there are several cost-saving benefits and capabilities to take advantage of depending on your unique business needs. By taking advantage of these benefits, or better yet, using two or more simultaneously, you can achieve significant cost savings while optimizing your business operations.
Click through for the list. None of it is earth-shattering, but they are still things to keep in mind along the way.