You need to maintain it at a minimum of 800k for 3 months after you get your extension and never drop below 400k until your next extension is granted at which point you can withdraw it.
Very easy to do it yourself and no getting stuck using agents for the foreseeable. If you are already inside Thailand you need to have between 15 and 21 days left on your current stamp to apply for the Non-O visa based on retirement, visit your local immigration office for their exact requirements as they all vary slightly. If your not yet in Thailand then applying for the Non-O before you travel is the best idea, especially as there will be no need for a return flight
You know what the solution is, you even mentioned it yourself. You are not a genuine tourist so get the visa that applies to your situation, 400k baht is not a huge amount of money and you don't even need to maintain it in the bank for longer than a couple of months. I'm sure you'll easily spend that in a year in Thailand
The Non-O, Non-OA etc are retirement visas and you aren't allowed to work on them. Get the DTV visa which does legally allow you to work remotely. Your husband can either apply for a DTV as the spouse of a DTV holder or apply for one of the retirement visas
You can't legally work on a retirement visa at all. There are only 2 visas that legally allow you to work remotely, the DTV and the LTR for Remote Work. People do it, but you need to keep it quiet
in total 15 months. 90 day Non-O visa plus 12 months extension of stay. It only happens if you use an agent. Do it yourself and you get 90 days Non-O visa then before that runs out you get the 12 months extension of stay