Threema is awesome. They even have a Mastodon account: mastodon.social/
Here are the settings I recommend during app and account setup:
• Tap next instead of making a Threema Safe password, and Tap Yes, you really want to continue without enabling Threema Safe • Tap next instead of entering your phone number, and tap Yes to confirm you don’t want to enter your phone number • Turn off “Sync contacts”
Make sure to scan each other’s QR codes in person to get your three green dots.
Since you have to pay for the app, it’s less likely you’re the product.