Yeah, SPA approach would separate the frontend and backend conceptually to different microservices.

Frontend is served separately off CDNs, backend isn't involved with it. When frontend team decides on a different framework/approach, they can switch, and the frontend just consumes the backend API.

Backend focuses on building an API, doesn't have to deal with templates etc. The other server side piece doesn't necessarily have to be Node, it can be whatever the backend team wants it to be, Rails is also fine. All it has to do is spit out JSON.