Ros and I did our big lap on our Red 2008 ST1300 in 2013 travelling just over 20,000 Kms in 90 days.
We did not go to Tasmania on this trip but had previously spent three weeks touring Tassie on an earlier trip did not feel so bad about missing them out on our Big Lap.
We rode on 60 of days with the other 30 off-bike days being taken at places where we wanted to stay at for a day or so and look around. On the riding days we averaged around 340 Kms per day which is a bit slower than what you are contemplating. This worked well for us given our age (late 60's) and the fact we were two-up. Your proposed riding routine sounds fine. We travelled Clockwise which suited us as we did the trip in April/May June. This allowed us to get the colder, southern part of the trip out of the way first up and still missed the wet season when we got to the top end. Either way (subject to seasons and temperature) would be fine.
Like Glen, we stayed as close to the coast as we could whilst staying on the bitumen. We had to back-track, a bit, a couple of times because we wanted to go to Darwin, Cooktown and Karumba but this was minimal and well worth the effort.
We stayed at pubs/motels/roadhouses and cabins.
We were fortunate enough to get an article published about our trip in the Australian Motorcyclist and I put a copy in my dropbox in case you were interested in reading it.
https://www.dropbox.com/s/2zv7d9zsg2bhziq/Round%20Oz%20Article%202013.pdf?dl=0We also ran a daily blog which you could check out here if you were interested.
http://www.bushellsbikebashes.com You would need to go back to April 2013 and work up from there.
We did not have any mechanical issues (thank you Mr Honda!). We had the bike serviced in Perth and Alice Springs and changed both tyres on the way. We tangled with a Kangaroo in Queensland which nearly brought us down and (we found out later) bent the front forks although we continued on oblivious at the time.
We maintained a daily log of costs, route and other pertinent stuff and would be happy to share this with you if you were interested.
We were fortunate enough to be invited to stay with a number of OZSTOC members along the way and they have since become lifelong friends. Don't pass up this opportunity should the opportunity come your way. If you need a bed in Sydney you would be welcome to stay with us.
We can only encourage you to make the effort to do the trip. You won't regret it.
Good luck
Dick and Ros