I am not expert and I would recommend reading an Apple Support article specific to iCloud and the Photos App - like: Set up and use iCloud Photos - Apple Support but I do know that as long as you are confident that all of the photos that you want uploaded to iCloud from your old Mac have been uploaded (meaning you’ve searched your entire old Mac for images and moved them to the Photos App and have confirmed they are there at iCloud.com - sometimes it can take a little while to sync newly added photos), then if you turn off Sync This iPad (Settings > iCloud > Photos (on) > iCloud Photos), and select “Remove from my Mac” (or similar on Mac), then all of your photos are still safe and sound in iCloud as this process removes/affects photos in the Mac’s Photos App only.
Yes, once you set-up your new Mac, and you turn on iCloud Photos and “Sync this Mac”, all of your photos (that have been uploaded) should download to the new Mac automatically. Ensure that you choose whether to have them “optimized” or not before you choose to sync photos to this Mac. 14,000 “originals” will take up A LOT of space on your new Mac.. Also, given the # of photos, the removal from your old Mac and sync to your new Mac both may take considerable time so don’t panic if you still see them on your old Mac or they don’t appear right away on your new one. It’s a lot of data and depending on your internet connection speed, plus, I believe, that there is also some “throttling” of icloud uploads and downloads so I wouldn’t be surprised to have the whole process takes 1-2 days or more to complete.
Apple actually makes it pretty tough to delete any folder or file from iCloud. The only action that you could perform on the Mac that would remove photos on-mass would be to click on “Manage Storage” - on the same iCloud Photos page as the “Sync This Mac” button - which takes you to a page titled “Storage” on which your can choose “Turn off and delete from iCloud”, which does and will delete photos on-mass from iCloud. However, even then you would still have 30 days to recover those photos if you made a mistake or changed your mind. Even if you delete photos from the “Recently Deleted” Photos App Album on your Mac, they still remain in iCloud (as long as they’ve been synced).
The only other way to delete iCloud Photos that I’m aware of is to have “Allow Web Access to iiCloud” turned on and then access your iCloud account on the web and delete files/photos from there.
Thought i’d mention, esp. managing 14,000 photos and that “I want to delete all my photos on my old Mac”, these additional steps may help in that regard:
- check the Photos App and ensure ALL albums including the “Recently Deleted” album (Apple created and yours) contain no photos.
- search your old Macintosh HD for “images”, “Photos” and, even file extension names like jpeg, jpg, png, heif, or any others you create/are aware of, and common image file names like screenshot, pic, Img, etc. e.g.) Nikon creates file names in a “DSC_0000” format by default so would search Mac HD for “dsc”.
- check any Apps that could possibly create images/photos e.g. Camera, Lightroom & Photoshop to make sure you haven’t forgotten to transfer these to the Photos App (or iCloud Drive through Files App) for upload or delete any photos you don’t want that were generated by these other Apps on your old Mac.
- Finally, I’d empty the Trash and, if MacOS allows it and you want extra reassurance, delete the Photos App from the Applications folder. This should remove all if it’s content and data (this may go to the trash bin after being deleted ??)
