What is the easiest and cheapest visa option for an Australian resident to stay in Thailand for six months?

Do it bit by bit, the everyday life may be far away from your dreams, so start out with a 60 day tourist visa, and see how it evolves from there.

Your rental has no relevance to getting your visa. The best for you would be a type O retirement visa, then the one year extension. Put bt800,000 into a Thai bank account in your name only, and just leave it there for how many ever years you want to stay in Thailand. The money is always yours, then the visa extension is bt1900 a year. You might not make any interest on the money, but, you will not have to do any border runs, or risk being refused entry for trying to live long term as a tourist. That is the cheapest. Others might suggest the type OA retirement visa, as you just have to show that bt800,000 equivalent in your home bank account, but, you are also required to have health insurance from one of the agencies on the immigration list, which is much more cost than interest lost. Not saying don't get insurance. It's simply better to not be required. The type O is the cheapest in the long run. Initially you get a 90 day stamp, then at about 30 days left you get your one year extension, and if you always come to Thailand around the same time you got the original 90 day entry you can do this for as long as you want to stay in Thailand for 6+ months a year. There are other ways, but they have more risk, or are more expensive.

You could look at getting a multiple entry tourist visa that would give you nearly 9 months of stay in Thailand if you time your border bounces and extensions correctly.

A single entry tourist visa would give you 60 days plus a 30 day extension and then you can do border bounces and extensions to get you to your 6 months but run the risk of a denial of entry - but that’s at the discretion of the immigration officer so no one can say you’d be able to or not.