It looks like you have two separate questions here, but they are tied together.
The first thing to take note of is that the multiple audience feature only filters upon the report/dashboard level. So you can't just put everything into one report and filter out pages (Unless MS changes that functionality). Next, map out which groups are able to see which pages. Depending on how that looks you can either have multipe pages in logical groupings or everything as standalone files. Regardless, one file needs to be designated as a "Primary" file to house all your models, measures, and RLS. Once the first report is published, the other ones can connect to primary's dataset to keep everything properly connected and synced. If your project has multiple models, just repeat the process as needed.
Assuming each group will have a unique landing page, you would create a report per group--conecting directly to the dataset if metrics are present. Then when you create the app, build out each group, hide what they don't need to see, and add the users in. Just make sure not to add the users to the workspace as any workspace member can see all views.
