Fly! @manlyago
Australian motorways are so slow compared to Europe!! It is a never-ending journey. With 3 teenage boys in the back, tucked together, hell no.
It isn't a scenic route . Yes, there are a few spots if you want to stop, like Port Stephens, Byron Bay, but you need to get out of the motorway and drive quite a bit towards the Coast.
If you are staying in Manly for 10 days, between Curl Curl, Freshwater, Palm Beach, Shelly Beach , Collins Beach, ... you have your share of gorgeous beaches.
Make sure to plan some viewings/alternatives for wet weather as a plan B, we have had 2 years of La Niña, extreme wet weather and we have just received warnings we are in for a 3rd year. www.theguardian.com/environment/2022/jun/10/triple-la-nina-australia-may-face-another-summer-of-flooding-rains-us-expert-warns
If Niña is confirmed, you might want to move to the Western Coast. Plenty of lovely things to see/do. Careful up North (north of Bundaberg in Queensland and in Geraldton in Western Australia) , it will be stinger season (Box Jelly fish) and you can't go in the water.
Do not underestimate the scarcity of accomodation in Australia . It is crazy .