As you are not buying a condo as an investment ignore all the nay sayers. My parents (Australian) bought a Condo 10 years ago and visited twice a year every year ( as did we) and now they live there happily. And as you said They left it fully furnished with all their things in it every time they left and just returned when they felt like it
No you can't work in Thailand, you can work remotely for another country in Thailand, or go the "soft power " route which is Muay Thai or cooking with extensions every 180 days which would work for you as you would be returning to Australia. Then after you turn 50 get the "retirement" visa
Some will most wonโt. It is possible but youโll need to try a large number of branches. Some people have said high traffic areas seem to more likely to let you
Apply for visa in USA and then you apply for extension to stay based on retirement after you arrive in Thailand There should be be plenty of information if you search retirement in this group