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.
I've flown the Bangkok Amsterdam route so many times, one way Air France the other way KLM and my experience is the exact opposite of yours. I love the Dutch and I would regularly sit in the back having a drink and a laugh with the cabin crew. The French on the other hand were unapproachable.
I met a couple of backpackers here 20 years ago who are now multi millionaires who subsequently invested huge sums of money here and created many jobs. Unwashed backpackers indeed!