is there a way to delete all the photos shared to my iPad from my iPhone (done through iCloud), but keep all my original iPad photos?
This is trivial to do from the Mac version of Photos. I don’t know of a way to set it up on the iPad or iPhone, but once you have your smart Album, it’s easy to delete all the photos that match the criteria selected for that album. Note, this smart album syncs to your iPad and iPhone so you can take action there in addition to Mac photos. 

On the mac, once you have your Photos library established (you don’t need the originals - you can optimize storage on the Mac if you prefer) - use the new smart album control and specify all photos taken from iPhone (or you can even select which lens, which model of iPhone, front or back camera, etc..)



Unless you have an import in the albums that contains all the photos you wish to eliminate, go for the computer to do the large curation / collections and deletions.

Smart albums are my secret photos power, list files not uploaded to iCloud to find errors, list pictures in date ranges or other specifics. Metadata is the key to unlock all sorts of awesome on MacOS and soon iOS with the new scripting being introduced there.