It's a fact most cloud providers' T&C's provide for access to your data to "ensure the terms are not being violated or a crime being committed" or whatever they state. So unless you roll your own Nextcloud hosting, if you are using commercial cloud storage like Google Drive, Dropbox, OneDrive, etc you can use Cryptomator on Windows, Linux, macOS, iOS and Android to provide your own encryption key to protect your data.
There is no vendor lock-in as you always have access to this source code and you can use any cloud provider you want to. It works transparently so no need to change the way your offsite backup works. It operates as a virtual drive and is much the same as if using a USB flash stick.
#cryptomator Cryptomator - Free Cloud Encryption
Free & Open Source client-side encryption for Dropbox, Google Drive, you name it. Protect your cloud files. Free software. No accounts, no backdoors.