Completely agree with you. create-react-app and friends are great for rapid prototyping but I learned a lot more by moving away from scaffolding tools & gulp, and setting up my own series of scripts and webpack configurations for dev and production. I also have to catch up on webpack2 changes but I'm a little hesitant to commit to a code-splitting pattern until I see the final details on the import proposal (looks like we're getting async await of imports - nice!)
My projects are looking very similar to yours these days, I've found jest and react-test-renderer very accessible for testing. Going with Mocha?
Yeah, I tried to use tape at first, but it was just too slow.. about 5x as slow as mocha, and that's without any actual tests in place. Right now all it does is hit the main test which loads all the js files (force coverage), all the others are empty... tape/tap took like 30+ seconds, compared to about 2 with mocha. Haven't touched jest though.
Not so much going for code splitting as separating the latest FF/Chrome (maybe edge) from the rest... since most of ES2015-2016 is supported in the browser, no need to transpile those features. So the bundle size can be quite a bit smaller.
One more thing I'm doing with the final output is building with preact instead of react, though may revert that in practice, since preact uses real-dom, which may be too slow for some use cases.