There are to two very simple methods to turn-off your iPad without using the physical buttons of your iPad...
1) Using the Control Centre
Swipe-downwards from top-right of the iPad screen to expose the Control Centre; tap the button at top-right:

2) From iPad settings
Settings > General > Shut Down
In addition to the two easy methods outlined above, you can use Siri - although, arguably, leaving Siri continuously enabled to respond to voice commands carries risk of inadvertent activation by you or someone else.
Using Siri
While Siri can be commanded to shut down your iPad, Siri must first be activated before the speaking the verbal command. To enable Siri on iPad, open the Settings app, select Apple Intelligence & Siri, then Talk & Type to Siri and select your preferred voice activation method, such as "Siri" or "Hey Siri".
To shut down an iPad using Siri, simply activate Siri and say, "Hey Siri, shut down my iPad" or "Siri, power off my iPad". Siri will ask for a confirmation, and once you confirm, the device will power off.