I’m not familiar with Caddy at all - I use Traefik for a reverse proxy, and my knowledge there isn’t huge either. But I think that your reverse proxy terminates TLS (HTTPS) from the world and then forwards traffic to the appropriate service on your local network using HTTP by default - but if your local service can handle TLS, I think you can configure your reverse proxy to forward the traffic to it using TLS.
I’m not familiar with Caddy at all - I use Traefik for a reverse proxy, and my knowledge there isn’t huge either. But I think that your reverse proxy terminates TLS (HTTPS) from the world and then forwards traffic to the appropriate service on your local network using HTTP by default - but if your local service can handle TLS, I think you can configure your reverse proxy to forward the traffic to it using TLS.