A little not so related question, what is/are the most used stack for web applications in Go? How common if at all would be a Go backend & React for instance frontend stack?

I can't tell about "how common", but a react web client with a go backend API is exactly what I do on my main product, currently.

It still has a few problems I haven't managed to solve, though (but not related directly to go). For example, given the visible routing is handled by react-router, I haven't find a way yet to issue proper 404 status (I have a catchall route that renders web client page, then client router displays a "not found" page, but it will be a 200 status). Not that a big deal, because the web client is an application behind auth, and not some public facing pages.

I suggest you adopt a 3rd party router, rather than using pure Go standard library for this. https://github.com/julienschmidt/httprouter is the most popular one. It supports a custom 404 handler https://github.com/julienschmidt/httprouter/blob/master/rout...