Heck, F Droid has Meshenger but it barely worked. Firechat and bridgefy are closed source so therefore compromised, so this is not an easy solve.
However, you can still download it from https://f-droid.org/en/packages/org.servalproject/
RE: Briar, I've tested it before. It's a promising app idea, and generally has good UI, but I ran into a few issues I'll post for consideration/refutation.
My results were hit or miss as to whether messages would get delivered if both apps were open, one was open, one was closed, one or both were online, offline, or a mix. It's unclear as to whether it is supposed to work 'online' via Tor nodes or not, the documentation is confusing. Getting an answer to the question "will this work over normal cellular data?" has no definitive answer.
[1] - http://p2pkit.io/