Yep. WhatsApp is better and Signal is better still, but iMessage's level of privacy and security is more than enough for someone who isn't specifically targeted by a nation-state-level actor.

it is critical to note that you also need to disable backing up your iPhone in iCloud to defeat having your key stored in iCloud:

https://support.apple.com/en-us/HT207428

It is also critical to note that everyone you communicate with needs to do this too, or Apple can still read your messages to them.

Any time others have access to your keys the only logical assumption is that they already know. Relying on goodwill is just naive