This FEP is mainly describing existing behavior, and aims to be compatible with implementations that do not have group-specific functionality. I understand the appeal, but one goal of the Mastodon proposal is to propose a distinctive UX for groups with different restrictions on posts, so that requires a clean compatibility break, which this FEP does not provide…
Otherwise, this seems pretty sensible, although I’m surprised at the use of Page for top-level posts (but that seems to be non-normative?)
Listing top-level posts is interesting, but it being in a replies collection sounds weird.
Curiously, the moderation actions do not seem to include deleting a group post.