You can get 6 months maybe more depending on what the immigration person thinks. But 6 is possible. You get 60 days on arrival visa exempt. Towards the end of that go to immigration and pay 1900 baht for 30 more. Then leave and reenter again for another 60 plus 30. 6 months.
So you’ll need 6 months. You could arrive with just a passport and you’ll get 60 days visa exempt. Then extend it another 30 at the immigration office for 1900 baht. Then take a border trip to Lao or Cambodia and on reentry get another 60 day visa exempt plus add another 30 for 1900 baht and you’re at 6 months with no visa. In case someone asks about your return you can book a bus leaving Thailand for $12 online and if asked show the bus ticket.
..lol. You’re close but don’t expect them to be productive. Most that age have Social Security or possibly a pension to live on and any money they do make is a bonus.
If you’re married to a Thai, then one option is a marriage visa.
It will require having 400,000 baht in a Thai bank.
(A retirement visa will require 800,000 in a Thai bank)
Another option might be the new DTV visa for digital nomads that give you a renewable 18O day visas for a 5 year period. You have to have 500,000 baht but it can be in your home countries bank…(collecting interest)….the DTV has to be applied for out of Thailand. Just log on to your countries Thai embassy website to read about visas. Please note that these rules can change without notice so keep that in mind. Good luck…🙏