Internet safety and how not to be doxxed

How about Riot? It’s got an app for iPhones and Androids and also available for desktop.

It depends what you're trying to achieve. If it's a small group of known individuals, it's relatively straightforward to create a private forum using freely available tools and a server. If you're trying to allow more wide reaching (in terms of users) discussion, I would say that you're better off keeping it fully public to encourage good personal information hygiene since an interloper could readily join and then scrape all the data.

Ultimately, any discussion forum will be a tradeoff between a mixture of ease of joining/participation, ease of coherent discussion, freedom from disruption and privacy.

I'm happy to elaborate on both implementation models and challenges.

I know a couple of people who used to run their own online forums. As in, the tech already exists for people to do it from a template. I'm afraid I don't know any more than that.

Security-wise I think there are various tools you can use as an admin, and get people to create a new anonymous email account and use usernames not their real names. If I we running it I'd want to be able to say that nooo I couldn't identify X user, this is their email address (if forced by law to give it) but that woulds be a dead end.