- Can Caddy work as a reverse proxy to other backend services?
Yes. Use this in your Caddyfile:
proxy /api 10.20.30.40:8080
No, QUIC is not supported at the moment: https://caddyserver.com/blog/implementing-http2-isnt-trivial
- and plain TCP for the front-end?
Yes, if you mean HTTP/1.x. Also, you can serve static files from the proxy itself. Everything not mentioned in explicit "proxy" directive would be served from local files in the same directory as Caddifile.