I started using hangouts, WhatsApp, signal, FB, and just yesterday line for a single person. This is really not a good excuse. People should be more important than tech.
None of these are an alternative to iMessage. They all either are missing key features or require you to deal with unencrypted communications or relayed encryption.
Regarding the key - you have to trust the app it doesn't share it with the app authors. That's regardless of who/how implements the encryption. And it's true for iMessage as well.
Are you sure about that? I was under the impression that, according to the security white paper released by Apple, that the key is generated on-device in the Secure Enclave and that Apple never has access to it. The Enclave itself also doesn't have network access so there's no way for that to be transmitted anywhere by any other means except for physical access, which is also limited in the hardware.