A quick check if this plan sounds feasible:
I came to Thailand 25 years ago, when getting visas was easy :) In the last few years, I've been working in Myanmar, and passing through Don Muang several times per year, on my way to my 'home base' in Cambodia. I never had any problems with Thai immigration, because I always had an onward ticket from Bangkok to Siem Reap, with just a day or 2 in Bangkok to go shopping etc.
But now I've just retired from teaching in Myanmar, and I prefer to stay in Thailand, because I have a Thai son etc. I'm British and 67 years old, but do not have 800,000 baht for a 'retirement' O visa. I do not want to use an agent!
I have an overseas online business that pulls in about $2,000 USD/month, and I also have $1,000 USD from my UK state pension. So a regular monthly income of about 100,000 baht.
I could live in Cambodia (I have an annual ER visa, but that's plan B). So I need to save up funds for either the retirement visa or (because I have a provable online business/steady income etc), a DTV visa which only needs me to save 500,000 baht.
But how to stay in Thailand while I'm saving up this money? I'm here now on a 60-day visa-free entry, and I could extend that for another 30 days.
'Dr Google' suggests that I register for a 10-month advanced Thai language course, and obtain an ED visa. I already speak Thai very well, and my local language school says that they can obtain a 10-month ED visa for me to learn/read advanced Thai etc, studying 3 hours/week for 40 weeks. I'm happy to genuinely attend such a course :)
Does this sound like a good plan? I will have saved up at least 600,000 baht in my UK bank account by the end of the ED course, and can hop over to Hanoi to apply for the DTV for my online job.