We have just done the WA coast without a porta-loo and I survived even being pregnant

The National Parks have very clean long drops and some even have flushing loos. Compared to other states National Parks, the long drops hardly smelt! Most also have park hosts that clean the facilities every day and some get regularly pumped out.
We didn't do Ningaloo Station or Quobba because of the requirement to have a porta-loo.
We made sure that we stopped at the roadside stops that had facilities, and I was really surprised at how clean they were. If we stopped somewhere without facilities we just used a rubbish bag to collect the toilet paper and buried our waste deep.
Only issue we had was irresponsible campers dumping the porta-loo contents into eco long drops and making them stink

Enjoy the trip, it's amazing!
Mrs Delux