We're about to launch a new site where one idea is to place a landing page with only static content and a signup-button on the root page (e.g. https://www.domain.tld/) and all other content not linked at all unless a user is registered and logged in. That other existing content would be properly listed within some sitemaps and ready for indexing.
I tried to find out what Google thinks about such a setup but had no success (yet). E.g. does it affect ranking and/or indexability as such of our new domain/root page?
Edit: All other content is not behind a login and accessible via a direct page request.