Have a look at threema:
https://threema.ch/en/It's quite popular in western europe. Unfortunately no open-source, but AFAIR the code was security-reviewed by an independant expert.
Yeah, this one seems all right, a million downloads in the playstore, no data storage/mining by provider. But, like you said, it's not open-source, nor is it p2p, hence not 100% private. Independent experts and service providers can be bought/coerced into disclosing users' personal data. It's doubtful that anyone will bother right now, but when and if it grows, it can become a vulnerable target.
Another problem with it is that it's not free, which means that it won't enjoy a similar level of natural growth that free counterparts, like WhatsApp or Telegram do, and I can't just ask all my friends and colleagues use it. So it's either going to remain on this unpopular level, where I can't really use it, or it will grow and with it will grow the probability of gvt. agencies tapping into it.
Really? It's 1.99 for lifetime use, even whatsapp charges you 0.99/year after the first year!
You're right about the closed source though, that is a showstopper IMHO.