I assume that you are not using iCloud Photo Library or all of your photos would appear on both devices. You also make it sound like you do not have a computer since you said that you are using the iPad Pro as a laptop. In the event that you do actually have a computer running the latest version of iTunes, my suggestion is to import the photos to your computer. Place them into a Photos folder on the desktop and then sync that folder of photos to the iPad Pro.
This will explain the steps to import the photos from the phone to your computer. You should be able to delete the photos off of the phone after you import them to the computer.
Import photos and videos from your iPhone, iPad, or iPod touch - Apple Support
This explains how to sync those photos to the iPad Pro. See the section titled, Sync Your Photos Manually Using iTunes.
Keep your photos safely stored and up to date on all of your devices - Apple Support
After you import the photos, if you find that you did not get the option to delete them at the end of the import process, you can delete them this way.
- Launch the Photos app.
- Tap the Camera Roll album in order to open it.
- Next tap Select in the upper right corner.
- Now tap the bottom right photo in order to select it.
- Lastly, tap the photo to the left of that first selected Photo, and slide your finger lightly all the way to the left and then up the screen until all of the photos have been selected.
It might take a few tries until you get the hang of batch selecting, but once you do, it really is an effective way of selecting and deleting the camera roll photos and it only takes a matter of seconds in order to select hundreds of photos this way. After you have selected the photos, tap the trash can can icon to delete them.
Don't forget to delete the photos in the Recently Deleted album in order to permanently delete all of those photos.
If you don't have a computer and you don't want to use iCloud Photo Library,you can use a WiFi transfer app in order to move the photos from the iPhone to the iPad. I use this app for that purpose.
http://www.wirelesstransferapp.com/
There are other apps available that do this so you can search the App Store to find one that suits your needs.
