If you want a community that is, at its foundational level, private, then you don't want it to be in the Fediverse. I don't mean it in an empathetic level but rather on a technical level. There are far better tools such as forum services for small communities, and you would be not wasting the second most distinctive feature of fediverse software (that of federation).

As for where can you have such a community, that is better split in two questions: 1.- what software to use and 2.- where to host it.

Question 1 suggests the use of any of a myriad of forum solutions, suchas phpBB or Discourse, that can be hosted or even self-hosted; as well as suggest the use of some goof practices such as having a back-channel communication system between high-level collaborators that uses a different medium/platform (say, Signal).

Question 2 highly involves your threat model if you want to do something that might put you and other people's lives at risk, but does not really mandate or prescribe which solutions you'd pick for question 1 otherwise. As I have not followed your conversations that closely, the most I can do is to suggestlow-tier VPS platforms that offer wide-range services (everything from forums to videoconference to blogs) so that there is less of a chance that it gets blocked; or otherwise use some sort of local net mesh (eg.: radio).