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.
Is there a serious effort on this front? At the very least a mobile version of Electron[1] applications would be a great interim step: you just need a browser for the UI and performance-critical bits can be written in C/C++ for accuracy and speed. Ultimately a modular UI paradigm can replace the dependence on a browser but that is likely years down the road.
[1] Cordova used to be the model; Ionic's Capacitor project is far more advanced but targeting iOS and Android specifically.
I think getting people onto devices that support open and flexible alternatives is the first step to actually converting users to these alternatives. I won't switch to a platform that doesn't let me communicate with people I know, and I wouldn't expect others to do so either. However, I could get people to switch from a less convenient option (e.g. Signal on Anbox) to a more convenient option (e.g. Matrix for messaging) in time.