Have you heard of Rclone?
It might be an encryption solution for you.
Aside from being able to sync, it can mount cloud drives as mount points.
There are clients available for Mac Windows and Linux.
You can create a client-side encrypted link to the NAS. Then, As clients copy files to nas they will be automatically encrypted with a client side password. So long as everybody that needs to share those files is using rclone and the same password they can decrypt the files.
Rclone has a crypt feature.
Files and folders Will not be readable by anyone not using rclone with the proper password.
Rclone can run as a service on Windowa or Daemon on Linux.
I also use Rclone to keep my files on Google drive and Onedrive encrypted.
Here is the Android version of Rclone. It will be able to access the nas server and the encrypted files.