Since your library is Optimized, that is what is slowing down the edit/delete/view options because your computer's hard drive is only storing small thumbnails of your photos. The full size originals are stored in iCloud Photos and have to be downloaded each time you want to view/edit them on your computer. So, how much room you would save by moving the originals - which are already stored as part of your iCloud Drive account - on your hard drive is probably not as much as you think. I also have the 2TB account and the Photos Library on my iMac (external HD) is 710GB for about 85K photos. On my iCloud Account it is about 680GB (probably slightly less because the thumbnails and actual files that Photos needs to run aren't in iCloud but in the library itself I guess).
But, assuming you want to try, you would have to export all the old photos you want to remove to your Mac's hard drive, delete them from the Photos app and then upload them to iCloud. I use iCloud to store stuff from certain apps and my Documents folder. Whatever is in those folders is uploaded to iCloud, BUT if I delete them from those folders, they are also deleted from the iCloud - like I said it is SYNCING solution, not a true Backup.
