by “FOSS” you mean compatible with the core values of free/libre software?
This rules out Signal because: 1/ some of its server software is proprietary 2/ they dont allow you to communicate with “their” users if you want to run the server software yourself 3/ the prevented authors of free/libre software in the past to distribute their software (find a fdroid/signal thread) 4/ in practice they channel their users through their centralized servers hosted on AWS
(and that’s without evoking their questionable funding, and long lasting commitment to make all their users identifiable through phone number, 10+y after US generals declared “we kill people based on metadata”…)
Simplex seems to me like the one really ticking all the boxes.
usernames is just for users. it is just a display thing. Signal still require that you use a phone# to sign up, and that you keep owning and paying for that SIM over the years in order to be able to verify it at random intervals…
despite being a very anti-privacy feature (esp. from a US company, funded initially bu US gov, who still forces its users to have their metadata stored on a US cloud…), it is also very much anti-user as in many cases around me, people who opened Signal accounts with some SIM card some day later traveled abroad, changed life, etc… and one day were asked to verify their account. (this is in some case what prompted their migration towards other communication networks…)