After my other post I'm looking over some other possibilities to do what I'm trying to do. How To Recover End-To-End Encrypted Data After Losing Private Key?
If I save user data in server, in order to restore the data I should have the key to unlock the encryption which is not safe. If I don't have the key then I cant decrypt the restored data.
From some comments and posts I read, I can store all the data in local and Google Drive, iCloud can backup the app data for me. I don't know how that works, if someone can explain the steps that would be nice.
Even though I can backup the data in Google Drive or iCloud, how can I use multiple devices at the same time with accessing the same data which is saved in local or backed up in drive? For example, Facebook messenger, WhatsApp etc. (mobile app and web browser at the same time) I might try to share the data with a shared key or QR code or something but I don't know how effective it would be.