There are many "private relays". You can run one for yourself and family / friends or a company one, even you could have by department.

Each relay you can configure to accept / post notes about a specific subject, like SN territories.

The "pay to post" or "pay to read" is optional, because in a nostr relay you can have whitelists and blacklists. That means only pre-authorized npubs can read and write on those relays.