they have plenty of ways to invest easily getting 5-15% but you need to leave the money in the bank for 5 months then it can drop to 400k just start your money foreign transfers of 65k + every month for 12 and you can take all the money out of the bank 800k isn’t all that much to worry about
You can only do the 90 day in Thailand it’s the simplest thing you can do at an immigration office,bring your passport,your TM30 and fill out the tm47 form