The photos in iCloud will NOT be deleted when you stop syncing on your iPad. If you stop using iCloud Photos on all devices, they will be deleted after 30 days. This support article details the steps to accomplish that. It makes a special note to download the photos to your device first, but that is so you continue to see the photos on the iPad. In your case, you are not interested in the photos on the iPad and after your turn off iCloud sync you can delete them from the iPad and it will not affect any photos in iCloud.
How to turn off iCloud Photos - Apple Support
As a safety precaution it does recommend to make a backup copy of your iCloud Photos even when shutting off on one device. Even though it may not be necessary, I would never say it is OK to not have a backup. Since you have a Mac, you most likely have enough storage space, so the extra step that it suggests would be worth it.
Archive or make copies of the information you store in iCloud - Apple Support