Bumping this thread to mention something in particular.
Over the course of the last 4-5 years since I started lurking the Purism community forums (and before that), many “secure” Tor instant messenging applications have come and gone:
Briar (Android only), Cwtch, Quiet, Ricochet Refresh, and Tinfoil Chat are actively maintained today. There is also Briar Desktop (GitLab), which is planned to support devices such as the Librem 5, but it has x64 dependencies at the moment (libc6, libexpatl, libnotify4, and zlib1g), among other issues, and OnionShare, but it requires JavaScript to use its chat feature, based on the last time I read its documentation. Speek is sort of unmaintained, but you can make that call for yourself.