My business model is subscription based therefore I need to use a payment service that is practical for this purpose. Unfortunately, as far as I researched, such services are only available to US individuals or companies, however I am outside US. (Merchant providers, BrainTree, Stripe etc.)
[Q] What is the best way to incorporate in US for only this purpose?
Having helped more than 30 foreign clients launch businesses in the U.S., I can tell you that the only effective way to incorporate will be to use the services of both a lawyer and an accountant / tax advisor who have substantial experience working with foreign clients.
Even then, while incorporating and obtaining a bank account should be reasonably straightforward, signing up with a payment service will be a challenge - but potentially feasible, given the level of revenue that you anticipate.
Disclaimer: This information does not constitute legal advice and does not establish an attorney-client relationship.