Be very careful of who you talk to about investment advice here in thailand most of the financial advisors are unlicensed and unregulated to work here, they will promise you the earth and it's 100% safe investment stay away from them you will lose your money 💰 look at offshore bank account fixed terms some are paying 5 to 6% at the moment.
I think it might be best to talk to someone in your Thai embassy or consulate in Australia to find out what visa to get because they keep changing the rules here, I think the OA you need medical insurance and the non O you don't or it could be the other way round.
Get yourself a 90 day non OA multiple entry visa in Australia and when you arrive in thailand and find somewhere to live go to the immigration office and then you can apply for a retirement visa, very easy to do, you will need 800,000 baht in a bank account or proof of