ironspider:
So do I need to use something like a transistor to allow the higher amperage to draw off the 5v on the PCB (from the wall wart) and avoid the ATtiny85?
Sounds right to me. Depending how well behaved the wall wart is you may not need to regulate its output.
In case you're interested, I would have thought you could achieve the same PWM result using a 555 timer plus a couple of external components, but I don't anything wrong with the approach you're taking if you prefer to use a microcontroller.