However, 240,000 in your bank account from interest. Then subtract the agent fees 40k first year the 16k x9 ( using your and his figures)
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0. So in your best case scenario you are up 56k over the ten years. However when you factor in the overall trend with exchange rates. If you'd bought ฿800,000 10 years ago, it would have cost circa £
Vs ฿2000 (90 day) then ฿1900 (12 month extension) to do yourself. It's paperwork not rocket science, a no brainer if you meet the requirements and have the 800k in your Thai bank account.
Don't forget the two land crossing are per calendar year 1st Jan to 31st Dec.
The crossing you want to use doesn't have the best of reputations for being easy. Perhaps try and engage one of the visa agents that do border runs through there from Thailand.
You could take the sleeper Train south entering Malaysia at Padang Besar. No visa to pay for entry to Malaysia. Cross the border on the train. Hua Hin - Hat Yai - Padang Besar.