Are there any cons to this approach? For me, having to implement setstate manually, not having full undo/redo/time travel debugging as an option/not being able to rehydrate the app from any state snapshot, along with the cognitive overhead of keeping state in sync in two places, one of which is mutable, are the big downsides.

I agree with this. I've tried redux-form and just manually handling it with Redux but letting the form data live in the form's component state felt the most clean to me.