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 🇹🇠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 🇹ðŸ‡.
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.