Not sure what you mean by "tits up". Never had a problem in the last five years. I transfer 65-70k each month from overseas. The transfers are tracked. That's what immigration needs to see. I'm happy getting the extra AUD$4000 each year. Pays for a couple of overseas air fares! Plus I don't tie up 800k for the rest of my life. If it works for you, that's good, I've just got better things to do with my money 👍👍😀😀
The rules regarding bank accounts are tightening around the world. Thai banks have decided not to open bank accounts to those on tourist visas (which the DTV is), but those on long-term non-immigrant visas can do so
I switched from OA to O, so I never had the 800k lump sum, but I believe you have to meet the requirements of each separate year to ensure continuity, so you wouldn't be able to withdraw the lump sum until three months after you've obtained the extension based on 65k per month
Depends on how much interest you miss out on. As an Aussie with a Super Fund paying in excess of 11% per annum tax free (so 88,000 baht per year) that's way too much to miss out on, and tying up 800,000 baht for the rest of my life, and still have to bring in money to live on anyway. For me (and for many other shrewd investors) it's a no-brainer. The 65k wins every time!