Anyone can legally move to Thailand, marriage is not one of the requirements. Plenty of single guys legally move here because they enjoy the company of many women, not just one 😆
It's what's known as the "cash economy", it goes on around the world, people avoiding tax (especially small businesses) by doing work for cheaper prices by accepting cash payments. I'm surprised you didn't know about that? Lots of things in life are "technical", there's always workarounds. No big deal. If anyone is happy enough to WANT to pay tax on money they bring into Thailand, they're free to do that. It's called "freedom of choice". I'll leave that comment here for others. By the way, it's 180 days, not 183
There is a workaround. Start with OA, commence the 65k transfers as soon as you get here, get the free second year, and towards the end, do a border bounce, come back in on exempt, and apply for non-O at immigration office using (by now) almost two years of 65k transfers. Means buying health insurance, but avoids the 800k lump sum
Sometimes higher. It pays to do several checks when looking for good prices. In Thailand I find somewhere suitable, note the price, then my lady phones them for the "Thai price". Doesn't always work, sometimes they quite higher than Agoda, but more often than not it's lower
Agoda is a pain because often they'll have the lowest fare which doesn't include checked baggage. I gave up on airfares with Agoda because it's too inconsistent. They should stick with hotels, they're good at that!
Better take a look at the DTV, it's a tourist visa, it's 180 days! Please provide link to the DLT website and Royal Gazette in relation to the 90 days.