Does Braintree work with magento2 for automated recurring billing? Or is authorize.net our only solution?
Magento2 does not have any kind of recurring billing solution built-in. Magento1 had recurring profiles, but that functionality was dropped in M2.
At present, there are two third-party extensions (that I'm aware of) offering this kind of functionality for M2:
Full disclosure: I wrote the ParadoxLabs module. I can't speak to Magenest's solution in terms of quality or what payment methods it might support. Adaptive Subscriptions is the only one audited and available on the Magento Marketplace.
The ParadoxLabs module only supports Authorize.Net as of this writing (as you may be aware), but we have support for Braintree and PayPal PayFlow Pro well underway, and others planned. Please contact us directly if you'd like to know more or discuss this further.