Older iMac won't sync photos to iCloud

Getting ready to copy/transfer ALL files, but most importantly ALL photos from an older, dying iMac to an external hard drive. I've updated and upgraded our iCloud capacity and have turned on iCloud in the photos preference tab and it is also checked in the iCloud preferences in the system preferences.....I've not seen a "sync" option though and I'm not seeing the photo library coming up when I log into my iCloud acct. I did notice that I had syncing off on my iPhone SE which I'm also expanding onto the Cloud as I used to back it up on sed old/dying iMac....I can see that those photos are in the process of uploading, but again....I canNOT get the photo library on the iMac to sync over to my iCloud..... please help - thank you! My iMac is older than 2014...newer than 2010....