I am implementing an IAM solution for a platform where I require secure login and multiple levels of access control to different parts of the platform. I am using this tech stack to build an MVP (https://grandstack.io/)
Per the offical Keycloak documentation, a relational database is required for the persistent datastore, so neo4j would not be supported for that purpose:
https://www.keycloak.org/docs/latest/server_installation/index.html#_database
As for authentication and authorization, Keycloak would be integrated with the React frontend as well as any APIs that need to be secured.