I replied to your post on twitter but I highly recommend Gradualizer[1].

Discovered it last year and started using it. The refinement portion handles unions and guards very well. It's not perfect but I'm integrating it in our codebase with some interesting successes. I'm hoping more people use it on their codebase and can find any bugs they have with it so it can be improved further.

[1] https://github.com/josefs/Gradualizer