What are the risks of being denied entry to Thailand if I stay for 6 months with just one border run as a US citizen?

. If you get the 60 day visa exempt stamp and extend by a further 30 days at the Immigration office, Immigration class you as a tourist. If you then attempt to enter again on a visa exempt stamp, Immigration rightly consider that after on 90 day stay in the country that you are no longer a tourist but that you are attempting to live long term in the country. While there is no legal limit to the number of Visa exempt entries, once the Immigration officer believes that you are no longer visiting the country as a tourist he will deny you entry. 3. If you have already stayed for 90 days on a visa exempt entry, then in future to guarentee entry into Thailand, you need to obtain a long term visa suitable to the reason for your stay.

If you are doing this every year then your chances of being denied entry increase. It's not a good plan for living inside Thailand 6 months of every year

Why don't you get a 6 month multiple entry tourist visa from your own country before you come ?

Are you asking about staying with a DTV visa or as a tourist? Tourist are facing more scrutiny now with long stays using on visa exempt.

With DTV there is no issue

Without a Visa your chances are getting slim without the use of a border bounce agency