I don't know how many banks have gone broke in the USA or Europe, nor do I care.
Yes, in Australia, interest rates are over 5%.
Paying tax on your interest depends on one's tax residency and amount of money they earn, in total.
Yes, your figures are correct. 28,000 baht more than covers the cost of the agent, and then some.
In Australia they are guaranteed as well.
I do not trust the Thai government, but do trust the Thai banks, they just pay lousy interest rates, and MUST follow Thai government policy. Do you remember what happened in Cyprus some years ago, and "The Haircut." I am well aware that as a foreigner in Thailand I have no rights. The annual extension is no more than a 1 year tourist visa.
I love living in Thailand. The climate. The food. The nightlife. The activities on offer. The beach. The list goes on, but that doesn't mean Thailand is a good investment for my "hard earned money."
Yes, I use an agent, as do thousands of others do. There's a misconception that those who use an agent do so because they have not got 800k to move to Thailand. Many use an agent because their 800k makes more than what the agent charges back in their home country. It's always through bank interest, but also though managed funds and other investments.
There are also other reasons why some use an agent. For example, I know an old guy here in his 80's and with mobility issues. He has the 800k in a Thai bank, but due to ill health he uses an agent. Of course, he pays less agent fees because he has the money, but uses an agent for a different reasons. What's wrong with that?
I'll probably be on the golf course while you are waiting in a queue in an immigration office. That's fine. Each to their own. Just don't say your extension only cost you 1,900 baht when mine costs say 13,000 baht, because when you look at the big picture, your extension has cost you a lot more than 1900 baht, and more than the 13,000 baht I pay, and it's very "strange" of people to not see the bigger picture.