Other apps like Nextcloud, can only backup documents (those not in apps) and pictures, because there's an API for this.
iTunes backup is an option, but it's not automatic and convenient.
I get more and more amazed at Apples lock in tactics. This is why I own nothing Apple, and have complete control over everything in my digital world.
So your choice is use Apple software to make your backups, or....?
However, in this case, the point is that you can use Apple software to make a local backup (and you can enforce the "local" part by doing so offline), and then use whatever you want to encrypt and stash away the resulting files.