> So I am wondering if I can reduce attack surface by making "management" services (Keycloak admin console, the headless CMS admin interface etc.) accessible only to me...

The answer to this is YES. Of course there are a variety of ways to implement. In your case I would start simple with something like wireguard. Keycloak won't be easy to install and configure as a beginner. If your needs are simple, check out https://github.com/lldap/lldap for authentication (and user management).