@BreachCat Threema protects your metadata from third parties and collects as little as possible. Decent choice.

Signal's metadata protection is a bit better; it supports Sealed Sender and PFS, but also requires your phone number. Good choice.

Briar is the best option in terms of security and metadata protection, since it's decentralised, meaning that no server is involved at all. Though, that requires both sender and recipient to be online at the same time.