Was working at a Java shop once which used Keycloak as a central IAM solution. As an FE Dev, I was tasked to customize/style the login-page provided by Keycloak, and quickly faced what you described: Pretty heavily Java-based, even to edit HTML templates I had to recompile using a full blown Java/JVM stack.

As an FE dev without Java background, this became pretty difficult. But once we finished that with the help of some of the BE Java devs, it ran (and still runs) quite stable and also the KeycloakJS adapter I integrated was alright without much surprises.