If you find your project ambitious enough, I think there’s no better choice than Fulcro. It simply offers consistent, full-fledged features for serious SPAs.
People mentioned lots of benefits of Fulcro in the other clojureverse convo you posted, so I won’t repeat them here.
Regarding your other questions:
Fulcro template (and the official doc!) does offer sample code for authentication and authorization.
The Rum+Datascript doesn’t have a neat story for server-client side communication. Blindly copy all server side Datomic datoms to client is not safe. Libraries like posh don’t offer much leverage, you still have to worry too much about authentication and authorization. The level of abstraction at which datoms are is simply not intuitive for auth(z) problem. And datom filtering is not performant at all. Edn-query-language, the Clojure native version of Graphql, is the higher level of abstraction that works well with server - client communication. And that’s the language Fulcro speak!