Idea: this could be similar to a professional association like NPPA or ASMP, open to anyone who agrees to the Code of Conduct and Code of Ethics. I'm an NPPA member, and their Code of Ethics is one of the reasons I joined. All members must agree to it. https://nppa.org/code-ethics
Another thought… there are already local-area servers, i.e. for a city or regional area. Maybe that’s where local/independent journalists should register, rather than on a dedicated instance. The purpose of local journalism is to serve the local community, after all.
Moderation and security seem like the two biggest hurdles for something like this, assuming anyone would join.