I hadn’t yet thought through specifics like “friend-post” or “public-post” modes, or whether non-users would be able to view public posts without an account.
I think you are saying you will use a chronological-feed instead of an algorithic-feed (where you sort, filter, promote to be most-relevant) to display posts.
Without it you may have a very large social media site on paper, but each user only interacts with a small subset which defeats the point of a social media platform. You've effectively created a group chat service that technically doesn't prevent others from joining in, it just doesn't let others find the conversation they are interested in