I don't think it's a good idea to worry about retention later. If people are serious about anything worth doing, there should be plans.

Maybe we should figure out how this will be different from r/ph. If it's just the same, what's the benefit of migrating when you can get the same stuff from r/ph? Unless there's a clear answer to that, it will be hard to attract others.

Aside from a pinned post there, those who are still active there might want to directly target members who can create content or encourage engagements here. Members with bigger following or influence that can help attract others are good starting points. If you create interesting contents for them here (not available there), they might join, but of course everyone is motivated differently so learning more about them is important before you can influence them.

How this community is envisioned to be will affect the plans for retention of members. Aside from interesting contents, there should a certain way to treat newcomers to encourage engagement. I read somewhere that the insensitivity of old members keeps new members away. A group effort on a how to behave is needed.

I think this is a complicated project that deserves its own thread. Since this is a community effort, we should hear from other members as well.