SCUBA instructor or English teacher are the two most popular jobs for expats. The SCUBA instructor would be cool if your GF is somewhere near tourist ocean side town. From what I understand it's difficult to get hired as an English teacher after 50 (I taught for a couple years previously).
Now you're left with...
If I were you I would focus on acquiring remote work skill-set.
If you're wife is super wealthy a visa to the US should not be a problem. Otherwise, forgot it.
You're in an interesting situation actually.. If you're sure you want to marry your GF, you may want to consider marrying her in the US. I *think* a fiance visa is easier than a wife visa. I *think* with the fiancee visa the US government looks at your financials and such, with the wife visa, they look at hers, which is a little trickier because US government officials are not familiar with Thailand's customs and such.
Many people in this group discourage it, but I have had good luck with it... You can apply for a marriage eVisa online, from Washington DC (or whichever embassy serves your home state). Once the visa is granted you will need to do a border bounce to activate it. For proof of nationality use your passport, for proof of residence use your driver's license. I have done this a few times, a few for marriage visa and a few for tourist visa.
All that being said. I asked an agent once, how much was a retirement visa and how much for a marriage visa... 15,000 retirement, 65,000 for marriage visa. I have known a few husbands who could get a marriage visa but the choose different visa options... The marriage visa can be a pain in the ass depending on the immigration department that serves your area.
People discourage it, in this group, but I have had good luck getting eVisa while in Thailand and doing a quick border bounce to activate it. Otherwise, you will need to go to a different country apply for a different visa, wait there until it's granted, then return on the new visa. Probably some agents can help you figure-out the quickest options.
I have "appalling English grammar and spelling!" and I taught for a couple of years. I did a crappy job but I doubt my students missed out on anything. Teaching in government schools doesn't require you to teach (all students will get passing grade, regardless of merit), it simply requires you to baby-sit, make sure the students don't hurt themselves or the property, and look like a teacher -- that is all that is required.
I have been here 10 years and I know maybe 10 words in Thai. Knowing Thai isn't necessary but probably there are benefits. Personally, I find it better that I don't speak Thai when the police pull me over. It seems they don't really want to take the time to try to explain stuff in English, so the bribe remains pretty low. If they could explain that I needed to pay more for this or that, I might have to pay more.