Aside from the obvious flaws (Threema isn't secure: https://breakingthe3ma.app/), this is LARPing: the author wants to keep his chats secret, but he doesn't discuss why and from whom. This is amateurish, as your efforts should be defined by the threat model, not the other way around. How much effort is your attacker capable of? How much effort are you willing to spend on opsec, a notoriously hard and inhuman task?

There's a whole difference if you want to keep your affair secret from your wife, your small-time weed dealing from the police, your spy ring from the FBI, coordinating anti-Russian attacks in Ukraine, or a Chinese resident resisting the regime.