Photos will start a new upload to iCloud, for many reasons - essentially, if there is any doubt, if all items in the photos library are already in iCloud or not. For example, the Mac will upload the library again to compare it to iCloud, if you temporarily disabled iCloud Photos and turned it on again. Or if you restored the library from a backup, or if you moved the library to a new location. Or if you repaired your Photos Library. All this will result in a new upload too iCloud.
As long as the current upload has not finished, Photos will not download any new photos from iCloud. You have to wait for the current upload to finish.
Do not change any iCloud preferences on your Mac or try any repairs, because that will just result in the upload to start over from scratch and it will take just longer.
You could try to find out, if there are items in your Photos Library that cannot upload to
iCloud. See this document: About the status bar in Photos for macOS - Apple Support
The second section "If you can't upload photos from your Mac" describes two smart album. Create these albums and check, if they are empty. If one of these albums is not empty, check the format of these items, if they are in a special format. Are these albums empty? Did you recently import new videos or photos to your Mac?
Which system version is running on your Mac? Did you just install an upgrade?
