there are no requirements, and Thailand goes by the requirements when you initially received the visa. They have always operated that way. I'll use the ED visa as an example. Initially there were no attendance requirements, but as the abuse showed up, and got worse, they required it being extended every 90 days, with documents from the school, and certified by the ministry of education, that you were attending class, and making adequate advancement, even though it was a one year visa. There are other examples, with retirement visa extensions, and others, but Thailand doesn't change the rules on you until the visa ends, or you relinquish it, starting over. There are still a few people on old retirement visa extensions that are only required to have bt20,000 in the bank, as that was the requirement when they initially got the visa, and it hasn't lapsed. Idealistically your thinking is right, but Thailand operates on reality, not idealism.
just remember, if you get the DTV now, and changes are made in the future, you are grandfathered in, with those changes not affecting you for at least five years your initial visa is valid for. That is how Thailand operates. Currently they are researching, seeing how people are abusing, or trying to scam the system, and will make changes in the near future to address the abuses. This is the Thai way. They did the same thing with retirement visas, education visas, volunteer visas, and will do the same with the DTV as time goes by. Currently there are no after-issue requirements. The only current requirements are to get the visa. There is no requirement that the money stays in the bank. No requirement that you attend any classes you signed up for, or no requirement that you continue working remotely after you get the visa. Requirements will change as time goes by, and some changes are starting to take effect at different Thai embassies already, but the exact requirements when you get it now won't change for the five years you initially have it. That is how Thailand does things.
It's all about your passports, not the fact you are married. You apply separately, the same as if you are not married, at the embassy of the country you want to visit.
you are just a little misguided on the different business structures available in Thailand, and the requirements. A business set up under the BOI, board of investment, structure does not require any Thai employees, except a licensed Thai accountant, and you can own 100% of the company, with zero Thai owners. There is also another option if you are an American citizen, but I won't go into that in detail, but simply saying under that the only requirement is a licensed Thai accountant, and that can even be hired part time. There's another little detail most aren't aware of when trying to set up a fake corporation just to get the long term visa extension. If the company is not making a profit within three years that corporation is dissolved by the government, and closed, ending the visa extension.
with any type O visa extension you simply get a multi-entry re-entry permit and that makes it multi-entry for that full year the extension is valid. It is that simple.