According to Apple, you can have iCloud backup enabled while maintaining E2E encryption with their Advanced Data Protection option:
https://support.apple.com/en-us/102651#advanced
You do have to be sure to not enable web access via icloud.com
https://support.apple.com/en-us/102651#advanced
You do have to be sure to not enable web access via icloud.com
Much easier to use a 3rd party app like Signal.