Concern about deleting photos from iCloud but not from my iMac.

I use my iMac hard drive as my main storage unit for my photos (with several backups, for security purposes). I would like to put all these photos onto my iCloud account, primarily so my wife and I can access them remotely. I take photos both with my iPhone and a digital camera. I have about 20,000 photos in roughly 100 albums, and I want to preserve that hierarchy when I put the photos on iCloud.

Right now iCloud Photos is turned off but I did have it on (without really knowing) for about two years until just recently. As a result there are about 2000 photos on my iCloud account, and they are not in any particular album..

I’d like to restart with a clean slate, first removing these 2000 photos from iCloud and then uploading all my photos from the iMac, in their respective albums, onto my iCloud Photos account. My concern is that I don’t want to lose any of the photos already on my iMac. I plan to keep iCloud Photos on my iMac turned off and then remove all the photos from the iCloud account. However, is there any chance that when I reconnect the iMac to iCloud Photos, now with an empty iCloud folder, that these photos could be removed from my iMac?