There is only thing I can advise regarding marketing: Build the best Landing Page.
No matter how you distribute the app, you want to build the Website for it. In other words, Google must know everything about your product, and customers should easily find the trusted source of information and support.
The most likely, the web page for selling an app will include the app title and description, some screenshots, the list of features, and cross-links to pages like Support, FAQ, Press, and Documentation. Maintaining a blog is a hard task, but if you are planning to release other apps in the future and stay in touch with the most loyal customers, you should definitely consider it.
The most important thing here is to design appearance only after the contents. First of all, make a SEO-friendly web page using best practices for accessibility. Only then make it great-looking and define styles with animations.
Also, it is usually a good idea to collect emails from users who are interested in getting news and other updates about the app and your future products.
Design and development is the easiest part. Promotion is the hardest one. Make sure to have a marketing plan before you release an app, because Launch is the time when you get attention and coverage for free. Use it wisely!