It is a Thai law firm writing a Thai prenup covering a Thai marriage in Thailand. He was an American lawyer able to assist me in the process. He did what you said in your comment to this post. It was a Thai lawyer that accompanied us to the marriage registration.
I used a law firm in Bangkok. One of the partners was an American lawyer. He, of course, explained whatever I needed to know but in fact my assets were pretty simple and there was nothing complicated about the prenup. My American assets and pension income were excluded. This is true in the US too. Assets you bring into the marriage are still your assets. What you acquire together while married are joint assets. The prenup was in Thai and English and the Thai lawyers explained the terms to my Thai fiancee. The law office insisted on assisting with the district office wedding to make sure the prenup was filed correctly. We are still married so we've never tested the document.
Of course, this will depend on your wife's education & skills but there is little chance they would even know where to start regarding looking for assets in a foreign country.
Note regarding the 20,000 baht. You can order this from your home country bank or currency exchange if you plan ahead. Even though it is rare to be asked you are going to spend it here anyway. You’ve just saved an ATM fee.
Once they start processing your application the BOI staff are extremely pleasant to work with. I recommend applying in Thailand so you can work with them directly.
I don’t know the requirements to get the original 90 day visa but after 90 days you would apply for a 1 year extension. When you have an O visa you do not need insurance to extend that O visa for 1 year. In order to extend an OA visa for 1 year you would need to have 1 year of health insurance.
You mean the US embassy in Bangkok. I did this in April. Everything done by mail. It took 30 days for me. They changed the process for paying for the return envelope. Good luck with that.