> Sharing types between React and a Typescript backend is great DX for developing both in parallel and I've not seen anything that comes close to being as enjoyable. Being able to change the signature of an API endpoint and immediately having all places in the codebase where the change will cause problems flagged as type errors is wonderful.

Have you ever seen Scala.js? Same language on the frontend and backend, in a way that actually feels first-class and real. Type system that's as powerful as Typescript but also sound. Full IDE functionality if you need it.