Apple cannot read your messages when you do not backup them in iCloud. That’s because there are two ways of encrypting stuff. One in which _you_ hold the keys (iMessage, but also EN competitor EvonThink), and one in which the provider holds the keys (e.g Google Drive, iCloud backup). Of course, when you hold the keys the service provider cannot give you anything extra by processing your data on their servers, it all needs to happen on your device(s). That’s one of the reasons why you need a fast processor to run iOS, while you can get away with a low end android phone. A lot of Google services “take care” of your data. Anyway: you holding the keys implies that you own the data, so look for services that implement that model.