Glad to see someone wants to see the west coast!

Driving Perth to Darwin would be a great way to see things, but in my opinion you would need more than 8 days: its >4000KM without adding any side trips, I am not sure after 500KM a day of driving you will have the energy to enjoy the surroundings. So your first option sounds better.

As an example of some things to do I took my girlfriend on a road trip last time we were back in Perth:

Day 1: Perth to Jurien Bay. Side trip during the day to The Pinnacles.
Day 2: Jurien Bay. Pre-lunch swim with sea lions. Post lunch drive to Kalbarri.
Day 4: Kalbarri: Hike at Natures Window
Day 5: Kalbarri - Money Mia.
Day 6: In Monkey Mia, Dolphins, Quad Bike Excursion.
Day 7: Monkey Mia -> Coral Bay
Day 8: Coral Bay: Scuba/Snorkeling Ningaloo reef
Day 9: Coral Bay: Scuba/Snorkeling in Ningaloo reef
Day 10: Coral Bay: Day Trip up to Exmouth
Day 11: Mammoth drive back to Perth (we planned to split it over a few days, but i was getting sick of driving).

It was a great trip, but would have been hard to make it shorter, and we only got half way to Broome. Maybe a one-way rental to Broome to Perth might work?

When you are in Perth don't forget the Southwest. Margret River is worth the trip, for a 1 - 2 night stay, visit some wineries, visit Cape Leeuwin, visit "The Valley of the Giants".

Its a big state, and 10 days is a good start (and I do hope you like it enough to visit again), hopefully the above will help a bit.