I have an old iPad that has about 8,000 photos on it, most of them not taken with this iPad, but rather synced from a Mac at a time when iPhoto on Mac was still a thing (before Photos.app and before iCloud). The photos appear in a hundred or so albums, which correspond to the events they were taken at. They are not handily in one single album like "last import".
Now I want to get the photo library off of that iPad, either into iCloud, or Google Photos. It really doesn't matter which one, I don't mind paying for storage, I just want them backed up to the cloud so I can wipe the old iPad.
I tried turning on iCloud Photos, but it gives me a dialog that says: "Photos and Videos synced from iTunes will be removed. Syncing photos via iTunes is not supported when iCloud Photo Library is turned on. 8,642 photos and videos will be deleted". No, I don't want to delete them, I want to upload said 8642 to the iCloud!
I then tried the Google Photos app: I turned on backup, and it backed up 77 photos to my Google Photos, the photos that were taken with this iPad. I cannot figure out how to make Google Photos see the rest of the 8000 photos not taken with the iPad and upload them as well.
Next thing I also tried was using Image Capture to get the photos onto a Mac - but image capture also only offers the same 77 pictures (taken with the iPad) as Google Photos.
Photos.app on Mac only offers to import 77 photos that were made with this iPad, just like Image Capture.
The linked question does suggest to upload to iCloud manually through the website, which doesn't work, because when one tries to navigate to the page one is immediately redirected to the "set up iCloud on this iPad" page.
I do have a Mac with a clean install of 10.13.6 and the latest ITunes, but it's not the Mac that this iPad was originally synced with.
If I had the photos on the Mac, I would be able to upload to iCloud the 8000 originals(exported from photos app) from there.
I'm at a loss now. How do I tell either iCloud or Google Photos to upload the library that did not originate on this iPad?