My first "social media" was h2g2, and everything since has been a downgrade.
For what it's worth though, If you're the poster of the thought, the post along with responses are available to you forever. They just 'die' from the point of view of the collective.
Of course, I understand this doesn't necessarily speak to your specific 'needs' but it is an overlap I kept in because I do resonate with your desired approach.
Then send out the invitations at the same time, so they all log on at the same time, so there's conversations and interaction for everybody from the get-go...