It would be unwise to stop this syncing process midway, but all you should have to do is turn the switch off. Since you choose to keep the originals on the iPad, there 'should' be no loss of data.
Hold in mind it can take a day or two for all photos to upload to iCloud Photo Library; depending on your internet connections upload speed. Upload speed is often much slower than the download speed. Mine took almost 3 days to upload. But my library was also much larger than yours, and most of it was uploading from the iMac, not the iPad.
No idea how long it will take the storage in iCloud to free up. If I remember it right it keeps your photo library there for a while (30 days) just in case you made a mistake and need to recover it. But that's old news, from when it was still in beta.