The iPad iOS and macOS are very different and you cannot use the Photos App Library on macOS with the Photos app on iOS.
An iPad is NOT a MacBook/Pro.
macOS apps DO NOT WORK on iOS devices and vice versa.
An iPad, even an iPad Pro, IS NOT a replacement for a full laptop/desktop computer, despite what Apple appears to claim.
An iPad Pro is named such because of it’s higher end features and it’s integration with the Apple Pencil, but still runs the same ole’ iOS that every other iDevice runs.
iDevices, including iPads, are companion/complementary devices to a real, full blown computer.
Multitasking is severely limited.
iOS 11 has a very limited File system setup but it is STILL not the files system the Mac OS uses.
Adobe, has yet to produce a full or near full version of Photoshop for iOS.
Adobe still has Photoshop functions split up into multiple apps.
There are a couple of full featured image editors for iOS that give Photoshop a run for its money.
NO conventional wired hard drives work with iOS devices, like iPads. NONE!
You need iTunes on a computer to sync photos from iTunes to an iDevice OR with the aid of a regular computer, you CAN use special mobile drives to put the bulk library of images onto and these special mobile drives that have device specific apps to view and import/export photos/images back and forth on an iPad to has with other apps.
To solely use an iPad, in place of an full computer, you would need to use a combination of special mobile drives AND “cloud-based”, offsite data storage services, like DropBox, BOX, Microsoft OneDrive, Google Drive, Amazon Cloud Drive and similar services.
With iDevices, reliance on “cloud-based” data storage services is pretty much a necessity.
Also, you would need to purchase an iPad Pro with larger storage size that your finances will allow, since there is no real way to expand the internal storage of any iDevice.
Good Luck to You!