Another thought is the two parts of locking it down.

How much local configuration would it take to make it so that Lemmy doesn’t show any posts to someone not logged in (since you know that if its on the web even under a really funky hard to type name… someone will find it)? Do you want your vacation planning available to everyone (“I won’t be home from September 15th through the 23rd, so Cousin Dave is going to stop by each day to walk the dog…”)

The other part is specifically against fediverse (which would be the reason for Lemmy rather than some other system) for such a private community. It again gets hard to lock down… not so much a “what gets out” but also a “what gets in”. … oh, Aunt Jane and Uncle John have subscribed to swingers that’s hosted on a nswf instance and that’s not about playground equipment. Meanwhile Aunt Dorthy wants you to do something to prevent Cousin Charlie from seeing all that “stuff” in /all fore Carlie accidentally calls him Uncle Long John at a family gathering. … and now you’re going through pics and finding pictures of your aunt and uncle that you really didn’t want to see (and you’re going to be going vegan before you want to think about that turkey baster next Thanksgiving).

And while these are solvable problems… it’s a question of do you want to solve them?

Simpler solutions will work better that you expand into other things as the need arises.