I have my own Matrix & Bridges setup (just like Beeper but with too much maintenance). It's super easy to write bots that you can then hook into Signal, WhatsApp etc, because essentially you only need to talk to your Matrix server, and the bridges take care of the rest. Talking to Matrix is easy - there's a straightforward HTTP API.

The other day I connected the matrix-chatgpt-bot (https://github.com/matrixgpt/matrix-chatgpt-bot) with WhatsApp - it was very funny to see how people reacted.