Note, you can’t extend early on, so extend as close to the end of your 60 days as possible, and do it at the immigration office that your hotel is in, your hotel will have to get some paper work for you.
I’d save every last penny you can, save it all spend almost nothing in the states while you are there. And focus your energy on a career that pays you in USD but you can work remote in Thailand. 6 years is plenty to figure that out, but they will want you to have income, even if you have a million saved, they will be more focused on you Atleast making 1600 ish USD a month.