Have you considered a blog, or even a newsletter?
It's probably worth asking OP what their position on censorship is before suggesting this. It's shocking to me, but many in the tech community are very pro censorship (though they will call it "moderation" since it's not happening to them).
Also if a customer of OP's got big enough and was controversial enough, AWS could threaten their account if they don't terminate that user's pages. So even if OP is a free speech absolutist, it still may not be a solution to the problem of big tech censorship.
If I were to explore public posting on Haven, then censorship/moderation becomes an issue. Public posting is outside of the focus I want to have and this would just make it more complicated for Haven so I'm not planning to explore anything that lets you "grow your audience"--I'm focusing on private sharing with your existing community.
Short of that though -- if a private company like Facebook or Twitter declines to host your content, that is entirely their right. It's not a public utility.
Free speech means you can say whatever you like. It doesn't mean you have a right to a platform and an audience.
And host it where exactly?
I realise you're doing this primarily for people to share personal content, but there's plenty of people out there that want to share personal thoughts to a large part of the world but just don't want to get censored by FB and your solution could work well for them.