Also, the second time I uploaded it didn’t recognize all that I had uploaded already (99%) and started the process again.
That is to be expected and normal. When you stop using iCloud Photos Library and then enable it again, Photos will merge your Photos library into the existing Photos Library in iCloud. This requires a second upload of all photos and videos to compare them with the items already in iCloud. Do not sign out of iCloud Photos Library unless absolutely necessary, because it is expensive.
How large is your Photos Library? Even with a fast internet connection the upload will take a long time, because it requires the merging of the library into iCloud, not just a data transfer. For my library with roughly 50000 photos and videos it will take usually 10 days or more, without any visible progress.
You may want to check the upload in iCloud. On your iCloud webpage at www.icloud.com, you can see in the Photos.app, which photos and videos are already in iCloud.
If there is no progress for several days, the problem may be caused by incompatible items in your Photos Library. You may be seeing some problematic items listed, when you create the smart albums described in the support document, that Jeff_W.
has pointed out. If there is nothing to be found in the "Unable to upload" or "referenced" smart album, search your library for other items, that may be not supported by Photos for Mac - videos with unsupported codecs, audio files, graphics files, any files that have not been taken with a digital camera or have been edited in graphics editors. See: How to Weed out Legacy Media in Photos fo… - Apple Community
