Looks like you can't use stripe and Braintree payment providers together with cashier in the same app based on a few issues in the repos on github dud to the conviction between database schemas... this one is most detailed
Seems only possible workaround would be to run separate apps/microservices for each provider alongside your main app and integrate with the microservice.
I also wouldn't wait for stripe to support PayPal
https://www.kylembrown.com/stripe/does-stripe-integrate-with-paypal