Any way to turn off the head unit?

holding down the on button just reboots the head unit...

Lets assume that the display uses quite high amount of power from a computer point of view of constant 30 Watts (Apple M1 Pro peak power consumption on max load).
On an one hours drive it will then consume 30Wh or 0,03kWh
with other assumption of 20kWh/100km(3.1 miles/kWh) consumption while driving means that you lose roughly 150 meters (about 500 feet) of driving distance per hour because of that 30W drain.
In real world I would be amazed if that display/navigator consumes that much power so in conclusion it has little to none effect on your range.

On my Prius I used to always turn of the navi display while driving at night, because that light shining to my eyes while driving was distracting, so I would like it to have an display off option, but not because of power consumption.

Interesting take on the watts usage. That's considering the CPU, but you have GPU which is probably even higher when running the 3d maps interface, plus the display itself and other components.
But either way, I found a way. Settings > display > display on/off. Not very intuitive Toyota for something that is a one touch button on every other head unit I've seen.
Absolutely love the car though, very minor quibble! maybe they could do a double-press on the on/off button to turn off the screen.
Looking forward to the software update coming out in Ireland for battery % indicator 🙂