Hate to say it, but if you don’t trust Signal to read your texts… you’ve already lost - stick with Facebook Messenger.
I understand you’re being a bit overdramatic to make a point, but no, Signal and Facebook messenger are not the same levels of secure for the user. That’s not the same as “trust them wholly,” but surely you understand they are on very different ends of the spectrum.
Memes I share with friends on Signal and verification codes from my bank with my life savings require very different levels of trust.
Why would I give any app permission to read my messages, no matter how secure it claims to be?