My wife is Thai and she travels on her American passport. We visited Thailand ๐น๐ญ from the USA ๐บ๐ธ many times over the last 15 years and never had a problem.
Plenty if you live within your means. With that amount you should even be able save some each month.
My wife and I are currently in Thailand ๐น๐ญ and have been here for 4 months. We budgeted 3,000$ per month and it has been more than enough including our shopping and nice dinners out once in a while. Best of luck to you!!!
I would recommend the retirement visa as you only have to report quarterly after your visa is approved.
I am here on a non o multi entry one year visa; however, we had to leave the country and return as each entry you get only 90 days even though the visa is good for a year.
Your flight can actually be early morning on the 20th however you must go through immigration before 11:59 as stated above. Once you go through immigration you are considered not in Thailand ๐น๐ญ.
What ever you do, do not go back out into the airport as when you back through security/immigration you will be an overstay if it is after midnight.