The major hole in your thinking with either the K-1 or CR1, in my opinion, is that you don't want to live in the US and fulfill the basic requirements behind the idea permanent residence in the U.S. You may want to review the actual residence requirements of a Green Card before going that course of action. Either the K1 or CR1 is much more costly and you risk losing the green card if you don't maintain residence in the US. As for the B2 application, this is one of those you never know until you try. You have heard correctly about the approval rates. When my now wife first applied for a B1/B2 (2013) she had a good job, owned house and land, and gave every reasonable evidence of strong ties to Thailand. Denied. The questions were mostly about me and did I have a job in Thailand -- which took me by surprise. A few years later, we both had a job at an American university in Thailand and they issued her an invitational letter. Approved. At renewal, they asked only her purpose for going and she told them it was to attend my doctoral graduation. When asked what university, the consular officer responded that he had graduated from there. Approved. Unfortunately, you would still become the focus of the questions. If you were employed in Thailand (preferably both of you), that could be enough to overcome the burden of non-immigrant intent. Either way, the B2 does appear the only reasonable option in the short-term to achieve your stated objectives. Based on what I've observed and read, it doesn't really matter whether you are married or not for the B1/B2. They will have questions about you and what you are doing in Thailand.
Should you decided to marry, there are some useful benefits in the immediate and whenever you do decided to apply for an IR1 visa. First, after marriage you can apply for an Individual Tax Identification Number (ITIN) to begin claiming her in your US income taxes. Second, when you are ready to apply for a IR1 visa, you will have been married for more than 2 years and become immediately eligible for a 10 year green card (less hassle and expense than a CR1 which gets a 2 year and must apply for removal of conditions). Whether married or not, you can get your child's Consular Record of Birth for citizenship, US Passport, and Social Security number.
We've waited until our daughter was 12 years old to apply for our Green Card and to move to the US for her school (7th grade). With you being in BKK, there are many great school options unlike in Hua Hin. If resources are not your problem, I can recommend NIST as an amazing international school.