On the contrary. It is extremely easy to convert from a 30-day tourist Visa exemption to a Non O and then a one-year based on retirement. And you don't have to prove the 800k came from abroad. I should know because that's what I did.
If you get a non O overseas you have to provide more information about finances including a background criminal check. If you get it in Thailand there are no such requirements. I converted my 30-day exemption then put the 800k in my account let it seed for 2 months and then I applied for the yearly extension based on retirement.
you are correct. The monthly pension income must come from overseas. However when I got a non O Visa inside Thailand on the back of a 30-day exemption entry I did not have to prove the 800k came from outside. It came from a Thai Bank. But in fact there is no financial requirement to obtain a Non O in the first place it is only when you use it to get a one-year extension based on retirement that you need to prove funds. And when I did that I just had to show the funds in the bank account no proof that they came from overseas. I've been doing this for 20 years.
You can go straight from a 30 day tourist extension to a Non O at Immigration in Thailand. Better to do it here than overseas. Then convert the Non O.to a retirement extentsion after seeding the 800k. That's exactly what I did.