Threema is the best. Reliable, Secure, Private. If Proton and Threema could collaborate that would be nice. Otherwise, stick with Threema. Signal (free) is a good second choice for to communicate with those who refuse to pay for Threema.

"Signal but better" what's wrong with Signal? Why create another messaging app? That makes no sense. The resources should be used to improve existing products, or create new products that are needed. It's hard enough to get people to switch to Signal, why split the userbase even further?

Yust use Threma

Because I like the proton environment : mail, drive, calendar, docs (when bug will be fixed), I feel like missing a messaging app. Lot of people still use whatsapp because they think they can't create groups with normal sms

there definitely are reasons for "why not", such as requiring ongoing development time that could go into Proton's many already existing products instead.
but also, what's wrong with Signal? or rather, what could Proton do better than Signal that would actually provide meaningful value and not just further fragmentation in the messaging app market? considering most people still just use WhatsApp or maybe Telegram because everyone else does, too, and cross-platform messaging does not exist to my knowledge