Chatty, the SMS messenger that works in Phosh also supports libpurple plugins, so presumably Signal could work alongside SMS (The SMS feature itself is a libpurple plugin IIRC)
I've used libpurple-signald with irssi and it works alright. It's not feature complete yet by a long shot, but much nicer to interact with than the Electron bloat.
This is a daemon that provides an API for signal
https://git.callpipe.com/finn/signald
This is a libpurple plugin for it (Pidgin messenger, or even irssi, Weechat etc)
https://github.com/hoehermann/libpurple-signald
Chatty, the SMS messenger that works in Phosh also supports libpurple plugins, so presumably Signal could work alongside SMS (The SMS feature itself is a libpurple plugin IIRC)
https://source.puri.sm/Librem5/chatty
I've used libpurple-signald with irssi and it works alright. It's not feature complete yet by a long shot, but much nicer to interact with than the Electron bloat.