It is possible to use a third-party App to backup photos (and other files) to a variety of external storage options - including USB storage, 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.
Connection of external storage devices is possible - with some caveats. Unlike USB solid state storage devices (e.g., USB Thumb Drives and SSD), Hard Disk Drives are notoriously power-hungry; unless your external HDD has its own source of power, you will likely need to use a powered USB-C hub. There are many USB Hubs that have a USB-C connection for iPad plus a USB-C PD (Power Delivery) pass-through power connector to which you can connect your USB-C Power Adapter - along with the hub itself incorporating a combination of additional USB-C ports, USB 2.0/3.1 Type-A ports, Ethernet LAN, HDMI, VGA, SD and microSD card-readers.
Perhaps consider Kingston, Anker, Hyperdrive and Satechi - although there are many others from which to choose. A Google Search for USB-C Hubs compatible with iPad will find a number of articles and recommendations.
To be recognised by iPadOS, the attached USB storage device must be formatted to FAT, FAT32, exFAT or APFS with a single partition. As the iPad has limited power capabilities, dependent upon the Storage Device, an external power source is often required.
Some additional information can be found here:
Connect external devices or servers with Files on iPhone - Apple Support
https://www.imore.com/how-use-external-hard-drives-your-iphone-or-ipad