Generally the flex is so integrated into the display that it's impossible to repair without damaging anything else on the display, especially with newer OLED panels, and that's assuming it'd even be possible to identify where the flex is damaged if it isn't a failed capacitor (which it almost never is). That being said, it's possible to replace certain parts of displays without replacing the actual LCD or OLED panel. LCDs are comprised of a backlight, LCD, polarizer, digitizer, and front glass. All of those can be replaced, some displays require specialized equipment in order to do properly, especially newer displays. OLED panels on iPhone and Samsung models have the polarizer and digitizer formed into the display because that's just how OLEDs are, but they can have the front glass replaced. You'll be needing special adhesive sheets called OCA, a vacuum press, autoclave, heat plate, and lots of practice. Older LCDs are so cheap now that it's generally hard to justify the time and risk involved in disassembling the panel. OLEDs are a different story, the glass is $5 and the display is $200-$300, not to mention how Apple likes to make replacing the display itself more difficult than it needs to be.

On iPad 12.9 2nd 3rd gen you can solder a new screen flex or daughter board. If flex is damaged from within the screen, then it's game over. You can separate digitizer from lcd and look further but that's not practical. It's very rare that visible components fail on ipad screens. I haven't seen it. White screen could be motherboard related. You have to check using a working screen.

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