@Canoodler You should look at thai insurance companies like "Thai Life" or "Muang Thai" and many others.
For example "Health Fit DD" plan from Thai Life allows new members up to 80 years old to sign up and renewable up to 99 years old. Plans go from 1 to 30 Million baht in coverage. For example 5 million coverage between the ages of 76-80 costs 74289 thb per year with 50000 deductible. (I found a premium table with google: image link)
or Muang Thai D Health Plus plan allow new members up to the age of 90 and renewable up to 99 years old. there is a calculator on their website here. It doesn't have a deductable option so it can get expensive the older you get.
And there are more options that allows sign ups to the age of 80 (Bangkok Life, KrungThai-AXA, ) and I think FWD allows sign ups to the age of 75. And I think Allianz Ayudyha has some plans too that are available for old people.
Quick tip: Even if an insurance company website has english version check out the Thai version, sometimes it has more plans. And not every plan is availabe at every age.
They all will be cheaper than Cigna.