Photos and videos stored on external storage devices do not appear within the iPad Photo Library - and cannot be directly accessed using the Photos App. To be accessible using the Photos App, images/videos stored on external storage must be imported to your iPad Photo Library.
Import and export photos and videos on iPad - Apple Support
As an efficient alternative to importing material to your Photo Library, It is possible to use a third-party App to directly access and manage files stored on external storage - including USB storage, third-party Cloud services and network storage. Of available third-party Apps, one App that comes readily to mind is FileBrowser Professional by Stratospherix:
https://apps.apple.com/gb/app/filebrowser-professional/id854618029
While this App will almost certainly meet your stated need, you might wish to review the extensive online Manual and help files to determine suitability for your requirements.
https://www.stratospherix.com/products/filebrowserprofessional/filebrowser-professional-user-guide.php
Speaking of personal experience, I am yet to encounter a file storage or file/photo backup scenario that the App won’t handle. I routinely use USB and Cloud storage for this purpose. File Browser also integrates directly with the native Files App, further simplifying file and photo management.