Are you rooted? Have you ever tried restoring a backup in this way?
Backup on android (without Google Services' online) is awful, SeedVault exists but has issues.
Am I missing something? Your phone isn’t using encrypted storage?
Encrypted storage is an implementation detail of the filesystem. Once you're at the point of connecting an unlocked phone to a computer running ADB, you can just transfer files and not care about whether or not the underlying data is encrypted at rest.