One of these is not like the others. I have a hard time trusting Meta's and Apple's claims about privacy, including E2EE. They're both billion-dollar corporations with a history of deceiving marketing practices and data leaks. Meta's business model in particular is based on advertising and abusing users' privacy. Why should we trust WhatsApp has their users' best interests in mind, when the company that runs it makes a profit from exploiting user data? To say that it's a conflict of interest would be an understatement.

Good luck with your app, but please don't recommend Meta and Apple products to users concerned with privacy.