Was starting to think about setting up a neighborhood Signal group, but now thinking that maybe something like Briar might be safer... only problem is that Briar only works on Android which is going to exclude a lot of iPhone users.

I spent a dozen years in SF, where my friend circles routinely used Signal. It's my primary messaging app, including to family and childhood friends.

I live in NY now. Just today, I got a message from a close friend who also did SF->NY "I'm deleting Signal to get more space on my phone, because nobody here uses it. Find me on WhatsApp or SMS."

To a naïve audience, Signal can have a stigma "I don't do anything illegal, so why should I bother maintaining yet-another messenger whose core competency is private messaging?" Signal is reasonably mainstream, and there are still a lot of people who won't use it.

I suspect you'll have an uphill battle using something even more obscure.

What about BitChat?

Why wouldn't you just use random abandoned forums or web article message threads? Iirc this is what teenagers used to do when schools banned various social media but not devices. Just put the URL in a discrete qr code that only a person in the neighborhood could see.