How should I sync smart previews ( not the originals) in a different way ? I believe I have first to understand this difference in the start previews workflow before I am able to manage all the rest !!!
You need to stop worrying about "syncing smart previews in a different way". Classic cannot sync a full copy of an original file to the cloud, it can only sync a smart preview of it. So when you tell Classic to sync an image file it first checks to see if a smart preview already exists in the smart preview cache....if it does, it simply syncs a copy of the existing smart preview to the cloud, but if it doesn't already exist it will create one which is synced to the cloud
but not retained locally. As far as Classic is concerned you have a bunch of images that are cataloged, some of them may be synced, some may not be. It does not matter that in actual fact only a smart preview was sent to the cloud, Classic just knows that its images are either synced or not.
The cloud apps work differently, in that any images imported into a cloud app will send a full copy of that original to the cloud....you can subsequently store smart previews of those images in the cloudy client apps by choice, but the cloud always retains the full original. But if the Classic catalog is sync-enabled, it will always download a copy of those original files....so you end up in the situation that Classic has a bunch of original images synced with the cloud (no matter if imported into Classic or one of the Cloudy apps), whereas the cloud could contain a bunch of original images (imported from a cloudy app) and a bunch of smart previews (created from originals imported into Classic and synced from there).
All you need to do, assuming you want to remove all originals from the cloud after import from iPhone, is do what we've explained in terms of removing them from the cloud and then re-syncing them from Classic. Let Classic worry about the need to create smart previews just for syncing purposes, you only need to be concerned about two things: first find a means to identify the images in Classic that are in the cloud as originals so that you can easily remove them from sync and then re-sync them again (plenty of ways to do that), and then including those images in your regular archive to the NAS. For the latter you need to do nothing different to your normal workflow other than to create smart previews of them before moving them to the NAS (athough Classic will create smart previews when you re-sync the images after removing them from the cloud, those smart previews are NOT retained unless they pre-existed). So just get in the habit of building smart previews prior to archive to the NAS (Classic will know if they already exist, in which case it will skip building them and will only build them for images which don't currently have one).