how do i turn on iCloud Photos back on after restore?

How do I get it to stay on and realize that the library it wants to upload is already on iCloud?

No, whenever there is the least doubt that the library you are trying to enable for iCloud may changed slightly, Photos will upload the complete library to compare it to iCloud, after restoring from a backup or moving the library to a different drive, or after repairing the library, even after turning iCloud Photo Library off and on again (Why is iCloud uploading my entire iCloud Photo library again after I moved its location?). The restored library from a backup may contain photos that already have been deleted from iCloud, or may have different edits and metadata applied. When Photos estimates the amount of iCloud storage needed for the additional upload, it will estimate the required storage using the worst case assumption that all photos are different.

You have now two options:

• Sign up for more iCloud storage for a month, so you can start the upload
• Or create a new, empty Photos Library and download the library from iCloud to your Mac. This will not require additional cloud storage, since the library is empty. This will work, if you did not import any new photos since the last sync with iCloud.

I prefer to download the the library from iCloud to a new, empty library instead of uploading the restored library. This method cannot restore the projects, but you can save your projects by keeping a copy of your current library on a external drive. Downloading to an empty library has the added advantage that it will be faster than uploading and merging into the existing library, and it will rebuild your library. my library has 50000 photos and videos. When I upload it again, it needs roughly ten days for merging into the iCloud library. When I only download to the empty library, downloading an optimized version will be done in half a day, and downloading all originals as well will be done in five days. To create a new, empty library launch Photos while holding down the options key ⌥. Keep holding down the key until the library chooser panel appears.

I just compiled this reply into a new user tip: If iCloud Photo Library cannot be turned on again because of lack of Cloud storage after moving or restoring the Photos Library

It still seems like a bug if the message is "You may run out of iCloud storage by uploading this library." but won't let you try anyway and hope for the sync to progressively reconcile.

Apple seems to be very cautious. If you try and the library will not fit into the available storage, you will be unable to use any iCloud services at all and it will be difficult to recover from that situation.