The progress bar is not continually updated.
But you should be seeing some progress by checking directly on your iCloud webpage in the browser. Open Safari and go to the web page www.icloud.com, sign I with your iCloud AppleID and open the Photos.app on that web page. Does the number of items in iCloud change, when you are looking directly at the webpage?
If the number shown in iCloud does not change at all, ensure that the cloud processes are running on your Mac.
It helps to restart the Mac and Photos occasionally to kick off the background processes.
And you may check, if you are items in your Photos Library, that are not compatible with iCloud. Is the status bar in Photos showing any items "On this Mac Only"? Is the smart album with the rule "Photo is unable to upload to iCloud Photo" empty? See: About the status bar in Photos for macOS - Apple Support
Some of the photos you recovered from an old hard drive may be corrupted or cannot be processed. Or you may have included videos and photos in an incompatible format. Which system version is running on your iMac?