You could also set up your passcode so it clears your phone after a few wrong attempts, then purposely enter the wrong passcode.
At best, physically crush your phone in a way that destroys the logic board.
Quickly press the power button 5 times. This makes it so you have to enter your passcode to use the phone (ie no biometrics).
The phone can probably still be accessed via rubber-hose cryptanalysis.
Starting with a full battery?
Not sure if this is possible, or if enough heat could be produced for a long enough time to ignite a battery fire, but that would be my approach.
Or just keep writing to the flash storage until you break down the oxide layer within the floating gate transistors. The phone won’t “destruct” but it would become inoperable.