Photos backup from iPad

iOS/iPadOS devices don’t natively support backup of files or photos to external storage devices or other Cloud services. Natively supported backup methods are to iCloud or to a PC/Mac running iTunes or Finder:

How to back up your iPhone, iPad, and iPod touch - Apple Support

It is possible, however, to use a third-party App to backup data and the iPad Photo Library to a variety of external storage options - including third-party Cloud services and network storage. 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.

There are some flash storage devices that might work. I haven't personally tried it. But there is the SanDisk iXpand. I'm not sure how well other USB-C flash drives will work. I think SanDisk has a specific app for the iXpand that can copy over files/images/video stored on an iPhone or iPad. Or a Mac. It doesn't sound like it's for anything other than Apple devices.

SanDisk iXpand Flash Drive Luxe USB Type-C Flash Drive 64GB - Apple

I have several computers (Macs and PCs) and I can easily just copy over all the images and videos in the Camera Roll. On a Mac I use Image Capture and on a PC I just use Windows Explorer to view the DCIM directories. That can be clunky though since everything is separated into different directories based on date. But if you don't have a computer to work with, then I'd think the SanDisk iXpand or something similar might be a reasonable option.