The general process is going to be to stop Nextcloud, move your files from your local instance storage to EFS, move the mount point, and then restart Nextcloud.
It will go something like this:
Mount EFS at a temporary directory.
sudo mkdir /mnt/efs
sudo mount -t efs fs-12345678:/ /mnt/efs
Stop Nextcloud
Change to the Nextcloud installation directory, then:
sudo -u php php occ maintenance:mode --on
Move Nextcloud data files to EFS
sudo mv -v /nextcloud/datadirectory/.??* /nextcloud/datadirectory/.?? /mnt/efs
Verify the data directory is empty
sudo ls -al /nextcloud/datadirectory
Unmount EFS from temporary directory
sudo umount /mnt/efs
Mount EFS as the Nextcloud data directory
sudo mount -t efs fs-12345678:/ /nextcloud/datadirectory
Set up the permanent mountpoint in /etc/fstab
fs-12345678:/ /nextcloud/datadirectory efs _netdev 0 0
Fix up ownership and permissions of the mountpoint and files if necessary
Restart Nextcloud
sudo -u php php occ maintenance:mode --off
You should change the usernames and directories shown above to match your own installation.