The power button was unresponsive, but I didn't manage to try for a significant time.
I had to leave it overnight and the power died which allowed it to restart again when I started charging it, but it then got stuck in a continuous loop of shutting down and restarting as there wasn't sufficient power to restart and I couldn't shut it down.
I ended up having to pause the restart on the initial warning screen which allowed it to at least charge enough to have enough power to continue the restart and complete the updates.
Been running GrapheneOS for about a year now and hod no problems until now, which has knocked my confidence in it a bit.