We’re rather short of any information that isolates the probable cause - such as the App running in foreground, or the Apps that are currently suspended (i.e., are visible in the App Switcher).
If having restored iPadOS (either full restore or DFU) the problem persists, perhaps the issue is not with iPadOS, but is instead an installed App. These may have to be systematically eliminated, perhaps by starting afresh and adding Apps singly until the reboot problem becomes apparent. Please remember, unless stripped-down to its basic configuration, no two iPads will ever be completely the same; your combination of Settings and installed Apps will be different from almost everybody else.
Try force-closing all Apps, force-restart the iPad, then observe the behaviour of the iPad. If the iPad is stable, open one App - and see what happens.
To force-close App, open the Task Switcher by swiping-up from the bottom of the screen - and briefly pause, before lifting your finger, to expose the currently open Apps. Swipe each App off the top of the screen.
When all Apps have been force-closed, force-restart the iPad:
- On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
- On an iPad with Face ID: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.