You are going about it the correct way. Get your non immigrant O in your home country. You can apply for your one year extension up to your final day of your 90 day stamp but obviously this is not advisable.
The problem with trying to open a bank account in Bangkok is that Bangkok immigration won't issue a certificate of residence until you have first done a 90 day report.
The process may be easier in your intended home base in Issan. Most immigration offices will issue a certificate of residence as long as you have registered your address by completing a TM30.
Armed with a certificate of residence, your passport with a Non Immigrant O visa and another form of ID (maybe a driving licence from your home country) you WILL be able to open a bank account. In my experience Bangkok Bank is the best account to deal with immigration issues and has the best English speaking staff.
If you run out of time opening the bank account and transfering the 400k you have the option as a married man to apply for a 60 day visit spouse extension.
not completely wrong. Once you use an agent it is hard to get away from using an agent. Some local offices won’t deal with you if you have used an agent. If you meet the criteria for the visa it is best to do it yourself.
Nick MuayPlam this is not true for all offices. I have a Thai ID and yellow book and I needed to go to my local Amphur and get a resident certificated from them. On the positive side it is quicker and easier than going to immigration.
Unfortunately all offices have their own requirements.
As other have said the Land Transport Office in Pranburi do requireuire a resident certificate. If you can get this in Bangkok then you can apply in Prachuap Khiri Khan.
The Pranburi office issue new licences two mornings a week.
Others have mentioned the video. You now watch that online and show the conformation that you have watched the video on the morning you apply.
absolutely correct. The OP has been on overstay since the day he finished working. With a minimal ammout of investigation by the Immigration Police he may find himself on a much longer overstay.
If I was to enquire, as you recommend, at a Visa Agency and they told me they could "clear an overstay" i would know that they were an illigitimate business and would definitely not engage their services.
If on the he other hand they booked me a taxi to the airport immidiatly and told me to report to immigration, pay the fine and leave Thailand then I would be getting good advice.