What you need is the App End-to-End Encryption - end_to_end_encryption
occ app:install end_to_end_encryption
You have to setup the encryption on your mobile device. Then you can enable it on your desktop client. Files are stored on the server full encrypted and cannot be decrypted on the server, even not by you or an admin.
You get a mnemonic of 12 library words when you create the encryption. That is the only thing you have to notice somewhere. Only with those twelve words you are able to decrypt the encrypted folder(s).
After you created those encrypted folders, you can disable End-to-End Encryption on your device / client, so nobody can see the content of those folders, even when they stole your mobile device or laptop with client. You can at any time enable End-to-End Encryption again on the client of your choice, by entering the mnemonic of twelve words.
I would recommend you to read all the manuals about it thoroughly. I use it myself and am fully convinced of it. It’s industry-standard encryption and privacy