So concerning the heat:
I got an 8 inch 5v fan setup blowing onto the heatsink.
I then spent an inordinate amount of time working out how to get the PWM working: https://forum.beagleboard.org/t/ai-64-pwm-confusion/32260
So I now have the fan working so it is pretty low and totally silent, running the two main arm cores at 100%, I am seeing around 40 degrees.
Running the fan flat out gives around 36 degrees.
So it looks like just a little airflow is enough to keep this thing quite cool.
I also worked out how to map the pin headers to the actual gpio: https://forum.beagleboard.org/t/ai-64-gpio-sysfs/32268
You might know all of this but most of it seems quite different to the old BBB.
I have managed to run stuff on the PRUs, today I am going to look at getting some gpio output from one of them...