Personally, I find the discussed messengers too heavyweight. I like the following two which have been around in F-Droid for quite a while already:
- silence.im (practically-serverless, uses regular SMS/text messages and encrypts them the signal way)
- delta.chat (practically-serverless, uses the existing e-mail infrastructure)
Both have their pros and cons, but I think integrating the silence functionality into the default messenger app on the L5 would not hurt. Of course it does not provide a solution for group chats and sending media, but would be way better than unencrypted text messages.