I agree with @Ethan9482 - you may need a USB hub, but also an Apple Lightning to USB3 Camera Adapter. If I understand your post correctly, the above two items should work to connect your iPad Air 3 to a HDD, or you may need only the adapter - see below. My comments below apply to your iPad Air3. If you upgraded to the just-announced new iPad Air, the connectors would be different. There is no need for an iPad Pro, in my opinion - your 6-month-old iPad Air 3 or the newest iPad Air should work fine.
This is the Apple adapter for your iPad Air3:
Link to above: Apple Lightning to USB3 Camera Adapter
This link from Apple Support is the definitive help source:
Apple support article - connect external drive to iPad
The above adapter allows connection of your iPad Air 3 Lightning Port to two USB connections, one of which could be a power source. One of the two ports in the adapter (the larger USB “A” port, on the left in the picture) could connect to your HDD. The smaller USB-C port would connect to power - you may need to connect this to a USB hub as @Ethan9482 suggested and then connect that to a charger, or you may connect the Apple adapter directly to a USB-C charger/cable without the need for a hub.
I would emphasize that this is only one way to do this. If you bought a new USB-3 SSD that connects to the USB-C connector, your power charger could then connect to the USB “A” connector in the Apple adapter. There are probably other 3rd-party adapters that are effectively “hubs”, with two standard USB “A” type connectors, or maybe a 3rd-party hub that would allow connection to your iPad Air3 lightning port.
I apologize for the apparent complexity of this explanation - We are in a state of flux with Apple’s changeover from Lightning to USB-C ports for connection to external devices.
There is another consideration - You mentioned that your HDD is 9 years old. Does it have a USB-3 connection? USB3 is preferred since it is much faster than the older USB2. Your 9 y/o HDD might be USB2, not USB3 (same physical connector, just a difference in speed). You might consider purchase of a new 500 GB or 1TB SSD that connects to USB-C. A 500GB Samsung T7 SSD, for example, is only $79 now. A 1TB Samsung T5 SSD is $139.
Hope this helps!