There's some work being done to make Signal work more natively on Linux (though not from Moxie directly)

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.