Just ensure you have enough money. I'd allow upwards of 500,000 Thb per year to live, plus 800,000 to be deposited in a Thai bank for your visa. Enter Thailand visa exempt and then apply for a Non immigrant 'O' visa when you're here. Simple as that.
I'm 60 with pre-existing conditions. Quoted $6,000 per year, excluding pre-existing conditions (which are the ones most likely to kill me).
Have set enough aside to hopefully cover at least one major illness/injury. Saving $6k per year, I can set aside 1 million Baht every 4 years for medical care in the future.