You have 8 days - and you think that’s enough time to get the ‘correct’ visa? When you don’t even know what the visa type is!
And no, you do not get visa on arrival. As has been stated hundreds of times on this site - you get visa exemption that allows you to stay for 60 days.
It is often to do with the economy. During tourist season, the Thai economy is quite buoyant. Tourism contributes around 20% of Thai GDP, so of course its a large influence.
It’s also affected by interest rates. And speculation. So, no not predictable or we would be rich. But there are trends it follows.
Well, it’s the ‘spirit’ of the thing. Do the Thai authorities create the DTV for the purpose it’s being used? I truly suspect they will change the rules to stop the abuse.