Go early . Take all documents that are usual and that will give you time to get any other documents needed. The only downside is you may have to redo your bank statement but that should only be 200 baht as it needs to be submitted within 24 hours.
I am the same age as you. I retired 3 years ago and so far have spent no more than £300 pound of medical care ,the majority of that was for a dental filling . I don't have insurance . My way of thinking if I need to spend three years of what an insurance would have cost me ,I will still be in credit .
Just another way of looking at it , especially if you are generally healthy.
Is a Non O not any good for you? You could get it in Thailand if you arrive with the tourist visa and convert . No need for medical insurance , but you need funds in bank