Best answer I've seen to this question. It's always important to understand what difference it makes to the bottom line. Often, for relatively small amounts (e.g., $3,000), it's not worth fretting over the type of slight move one is likely to see week to week. No one knows whether it's going up or down. Historically, in recent times 35.48 is a good rate.
a Mee Tae Dai savings account at Krungsri Bank is currently paying 0.90% interest earned daily paid monthly. Also, if you provide your Thai Tax ID there will be no 15% tax withholding on earned interest. Fixed deposit accounts have mandatory 15% tax withholding. This account is accepted by immigration as principal is not subject to market fluctuation and immediate withdrawal is possible with the only penalty being loss of future interest on withdrawn funds.
using an agent to circumvent the financial requirements involves your agent bribing an immigration officer to waive the financial requirements in your case. That is clearly illegal, but currently this is not enforced. It's similar to foreigners creating a company whose sole purpose is to allow them to buy land, which is otherwise prohibited. These things are illegal, but commonly not enforced. Not something I would want to be involved in myself.
Using an agent to circumvent financial requirements can result in immigration, who can tell how you obtained your extension, refusing to deal with you if you decide not to use an agent in the future. They will tell you to continue to use your agent. Again, not a situation I want to get involved with. The only entities I allow to hold my passport overnight are, my embassy when renewing, Thai immigration when I receive my signed retirement extension the next day, and a foreign consulate when applying for a visa. I don't willing give my passport to any agent, anywhere. A friend of mine had an agent keep his passport for over 100 days. That's not for me.
I don't find it overly burdensome to follow the rules.
4) yes, been doing that for nine years in Phuket. Never have to leave Thailand, but need a re-entry permit to leave Thailand (1,000 single, 3,800 multiple re-entry) if you want to return and resume your current permission to stay.
3) in some countries health insurance may be required for a Non-O, but when you extend that in Thailand it is not required. It is required for a Non-OA, but not for a Non-O.