Need to turn off USB charging in my iPad

You cannot disable USB charging of your iPad. Whenever your iPad is connected to a USB-bus that is capable of sourcing sufficient power, the iPad will charge. It is the power-source that determines whether or not to provide power - not the power-sink that receives power.

Unless you DJI controller has the ability to inhibit charging of an externally connected device, you’re out of luck.

If both your iPad and controller both have a USB-C connection that support USB PD (Power Delivery), the sequence that you connect the USB-C cable to connected devices will determine which device be becomes the power source.

to the best of our knowledge, it's not possible, which may be the reason of lack of answers

This is a user forum, you are not addressing apple here  just  fellow users

you can try contacting apple Support ->  Get Support (apple.com)

and or give them feedback -> Product Feedback - Apple

Re: “… or use a cable that only transfers data …”

That might possiby be a viable option in your situation; I’d think that this wheel might have already been invented IF it’s even technically possible.

Perhaps some other forum focused on USB implementations could be a source of info.