I think your general vibe is good here, but I don't think you need the posting limit, necessarily. Compared to Facebook, to make it less about re-sharing spam and the like, and more like a conversation, what you need is:
* More emphasis on text (which you already have, great)
* Less emphasis on photos. I hate how when you create a post on FB with a lot of text and photos, photos get much more space within the post layout, it should be reversed if you want a conversation, and if people want to see a bigger version they can always click them to blow them up.
* Links aren't treated as special. You can include them in your posts and people can click them, but they shouldn't result in including a photo+description on the post.
* No sharing other people's posts. You can copy+paste and link to their post (which will only work for other friends of them), but no re-sharing.
Especially those last two mean that you don't get the 'spam' of people sharing content from other people or websites.
Anyway, overall great job, this is definitely something I wanna see. Facebook is full of news spam/hot takes and the like, Instagram is largely just photos and videos, I really want something that's focused on what my immediate friends and families wanna talk about (not just share photos of).