I'd be pretty angry if I found out someone I chatted to on Signal was running a service to workaround my message expiry choice and archive my messages. And breaking that trust just to run it through an LLM?
Be angry then, IM communication is not a contract with terms unilaterly decided by one party. Or put a confidential signature with every message, see how well that works for you
Be a better person. Or at least put a message in your status that you record everything and send it to Google for analysis for minimum personal gain. Do you also carry a second phone to take a photo just in case someone sends you a disappearing photo?