You have a lot of time and a lot of miles already. Since you have no status on Delta, in your shoes, I would be looking for ways to get enough miles to fly Business. Without at least Gold status, travel to Australia in coach will be miserable. With Gold, you can get a pass that cuts you to the front of the line at customs, lounge access with showers (etc.), better seats, and so on. But for no status, I think you are correct to assume that coach is not an option. The mileage charts have been (and probably still are) changing, so assume you will need more miles than you think you will need and make a plan to bulk up your account. I do not think getting low level awards ex-ATL will be anywhere near as difficult as it is for those of us who don't fly out of a hub, based on some experimental searches I have tried, so your location may be an advantage to find the awards when you are ready to book.

There are lots of ways to get affordable hotels in Australia besides chasing chain hotels -- such as upscale hostels with private room for couples -- so that would be FAR back on my list of priorities if I had limited spend to use chasing points. In fact, staying in upscale hostels is what I did. There's also priceline, airbnb type equivalents, and so on. So that's how I would optimize my spend -- focus on the miles and worry about searching for hotel, B&B, or whatever bookings once I have tickets in hand and the dates firmed up. People who focus on these chain hotels always gripe about expensive SYD is but IMHO it's their own fault for not looking at how real Australians travel.