Probably a terrible idea but have you considered a private Lemmy instance? At the end of the day Lemmy/PieFed/Reddit are just forums with conversation threads and upvotes.

Lemmy is probably way more of a resource hog than the other various php options but from a usability standpoint if you have a favorite Lemmy mobile app it would work for your private instance as well.

There appears to be a private instance mode that disables federation.