Can I make a couple of statements that may help understanding your use of the 'Lightroom' apps-
1) Lightroom (Cloud based. Lightroom-Desktop on a pc or Mac, Lightroom-mobile on a iPad or phone, Lightroom-Web on a web browser.)
Stores all images you import from any of these 'Cloud based' apps in the Lightroom ecosystem 'Cloud'. These files occupy your storage space of 20GB. You can use any of these apps on any device to access your photos stored in the Lr-Cloud.
2) Lightroom-Classic (Local based. Installed on a pc or Mac) stores and uses all photo files on a locally connected hard-drive. Lr-Classic has limited syncing with those 'Cloud' photos because-
a) Lr-Classic will only sync photos UP to the Cloud as smaller proxy (Smart Previews). These files are suitable to edit on any 'Lightroom' device (with one or two minor limitations.). You can be selective for the photos to sync UP because they must be in a Standard Collection marked to 'Sync'. FIles you sync UP do not occupy any of your 20GB storage (so can effectively sync many photos without using storage.)
b) Lr-Classic will only sync ONE catalog to the Cloud. It can be changed to another catalog, but with complications.
c) Lr-Classic, when sync is 'on' will automatically download all photos from the Lr-Cloud. (Download folders defined in LrC preferences).
"added LR to my I-phone, Surprised to see my first 982 photos were available on the phone" -Yes photos you imported to Lightroom (Cloud based) are available on all Lightroom apps.
"I'd quickly run out of cloud space"- Then you will need to consider ways to reduce the photos in the Cloud.
"Do I need to delete the original photos (scans) from the cloud before syncing the I-Mac?"- When SYnc is turned 'on' in Lightroom-Classic all the files will download to the iMac catalog, so do not delete anything until you are sure you have all the Cloud photos downloaded to the LrC catalog and local folders. After that there are methods to delete files from the Cloud and Re-sync them back to the Cloud (without using storage!)
COMPARE LIGHTROOM & LIGHTROOM-CLASSIC FEATURES
https://www.lightroomqueen.com/limitations-syncing-classic-with-cloud/
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