Nope and there likely won't be for quite some time. Sorry, it's just not a priority right now.

JSON endpoints are simple to make. That's not the problem here. You need an authentication system in place to ensure the Rest API isn't being abused and probably a quota limit API to stop any app from overloading your site. You can make JSON endpoints easy enough just using CB Auto Actions and not specifying a Trigger then adding &format=raw to the action URL when using Code actions.

I also wouldn't trust any outside developer to provide you with quality work unless they have existing advanced experience with Joomla API at the very least but preferably also CB API.

Don't have any comments regarding joomlarestservices or moonsoft. I've no experience with React Native, but just React which is great so I assume React Native is also probably great.

You'd still need to have JSON endpoints for React to communicate with. The JSON endpoints you can do using CB Auto Actions if you wanted, but I'd make some sort of private key and condition against it in your action so it'll ignore any HTTP requests that don't have it. That'd at least be some basic security. You could even do an authentication exchange using CB Auto Actions and a custom table to keep track of the keys. So the only coding you'd have to do is the PHP for outputting the JSON response you're expecting from the endpoint.