No need to use an agent. Extending the visa in Hua Hin took me about 30 minutes in the immigration office this year. It's one a4 form with about 15 questions (name, address, passport number, date arrived in Thailand etc). Ask questions here if unsure and you will be okay. Opening the bank account is the most difficult part, but with the non-o visa you should have no problems. Good luck.
you must be in the Non-OA section to be getting that requirement. Non-O stands for 'other', so it's a bit of a catch all visa for anything that doesn't have a separate visa. The non-O must still be there otherwise the visa advice page would be full of Australian people complaining about it. The requirements for the 90 day non-o retirement visa are on this picture from the Australian site and it doesn't mention insurance (picture attached). If all else fails, maybe make a separate post to ask where it is and I am sure one of the moderators will direct you.
The Australian website is not clear, so I can understand why you are struggling to find it, but I would need to create an account to get to the page and I don't really want to do that. The top option in the picture I sent is the visa you need.
I got my own visa and have extended it in two different provinces several times. I opened my own bank account. I also got married, bought a house, got the yellow book and ID card. I also obtained my 2 and subsequent 5 year driving licence and bought a car. All of these things I did WITHOUT using an agent. The visa and extension is one of the easiest things to do (the extension is one A4 form with about 15 questions (name, address, passport number, date arrived in Thailand etc). If I am unsure how to do things I ask immigration or online here where there are some brilliantly knowledgeable and helpful people like the moderators. You quickly learn to ignore or block the idiots telling you false information or to use agents.
Ulrik is 💯 correct. When you apply for the Non O visa in Aus, you can use money in your Aus bank as proof, but when you apply for the 1 year extension in Thailand your 800,000 has to have been in the Thai account for 2 months (I think you know this already, but just confirming). The non O does not have a mandatory insurance requirement. It is the non OA that requires insurance and police checks etc. The only thing that has changed recently is opening a bank account. It used to be possible with a tourist visa, but now you need a long term visa (like the non O) to open an account, so you will be okay. Good luck.