Ironically the only auto reply bot app for Signal I found is closed source ...
Ironically the only auto reply bot app for Signal I found is closed source ...

www.getdroidtips.com
How to Send Automated Replies in Signal App?

... do you know any other bot app that can do auto replies and is open source?
Well, it might be kind of overkill, but basically you could use matrix to bridge to signal and then use a bot to do automatic answers.
The matrix ansible docker deploy playbook would be able to install everything you'd need for that:
All of those components are open source.
That’s really cool