Obligatory briar mention...

Briar supports communication over multiple mediums, including wifi & Bluetooth, has forward secrecy, and feels quite 'signal-like' so its not impossible to get people to use it.

https://briarproject.org/

Briar cannot do offline delivery unless you setup a stand-in service. Unfortunately that's way to complicated for regular use.

Briar looks really nice for people that really need the extra security and privacy, though.

I've been interested in SimpleX lately, but I don't really have anyone to properly test it with.