Too bad Android basically ties my persona to the phone, making it difficult and awkward to use this on more than one device. Is there a RPi version?

I don't know of anything that has SMS or signal messaging built-in, but the motion project¹ would probably be a good place to start.

Motion + signal-cli[0] has been working well for me.

[0] https://github.com/AsamK/signal-cli