PR requires unbroken extensions on a non immigrant visa for 3 years which isn't possible on this visa. You need to leave at minimum once per year and it's officially classed as a type of tourist visa.
It does sound too good to be true but all indications are they will accept people staying for five years, the government published material on it clearly states just need to leave at least 1 day per year for a border run and can come straight back.
should be possible to get a Thai tax ID, I know some retirees do it to claim back the 15% withholding on their 800k interest. I know others have got it because their home countries or home country financial institutions wanted it. My understanding, it's not related to visa type at all.
it would mean you'd have to renew after six months and leave after 1 year which you don't need to do on marriage. Possibly will be a one time showing off 500k though. You'd have the 40k monthly income option with the marriage visa too which needs no money tied up in the bank.
extension is a lot quicker. How much so depends how close you are to a border/airport. Certainly if extensions are 1,900 but even if they are 10k I could see people choosing to extend in country if they don't want to leave for some other reason.