The specific challenge with authz in the app layer is that different apps can have different access models with varying complexity, especially the more granular you get (e.g. implementing fine grained access to specific objects/resources - like Google Docs).

Personally, I think a rebac (relationship/graph based) approach works best for apps because permissions in applications are mostly relational and/or hierarchical (levels of groups). There are authz systems out there such as Warrant https://warrant.dev/ (I'm a founder) in which you can define a custom access model as a schema and enforce it in your app.