Your plan of starting the degree is a good one. Attempting a part-time degree later in life is much more difficult, particularly if you have a family. As Genghis implied, education, flight experience and maturity can only help in getting through military flight training. The last thing you want to do is join as a DEO pilot only to scrub-out shortly after with no back-up plan.
If you want to work as a civilian test pilot, then it will have to be outside of Australia where there are substantial aerospace industries. In that case you will need a degree to get a position. Also you will most probably need a degree to get a work visa, unless you have dual citizenship or marry someone that does!
I recommend you aim for a degree first. If offered a DEO pilot position it would be a tough decision to turn it down. However you would be better off in the long term with a solid education behind you.