I'm very surprised that Relay is not getting more traction in this space. Given, it's a lot of work to get it set up and running but once it's in place it's great. I haven't found a library that manages network state and the associated difficulties that come with it better.

No reason why you cannot combine both Redux + Relay, in fact once the app gets to a certain size and you want to rely less on passing around long chains of callbacks and state objects then you pretty much have to implement either Redux, Mobx or similar.