Take a look at Twirp (https://github.com/twitchtv/twirp) open sourced by TwitchTv. It's a lot lighter weight than gRPC. It does use Protobufs but addresses some of the concerns you mentioned, such as being able to test with JSON payloads, works over HTTP 1.1 and HTTP/2, good client libraries, and doesn't require a proxy.

They address your concerns in more detail in the Twirp release announcement (2018) - https://blog.twitch.tv/en/2018/01/16/twirp-a-sweet-new-rpc-f...