pretty sure you need multi-entry, which costs extra. no idea why so many people decide not to pay the small free for this, and end up getting their visa cancelled when they go somewhere, for even 1 day.
So many have got used to relaxed regulations, and endless border bounces,.. Thailand has announced for many months they're cracking down, then someone gets questioned, and "Oh my! The sky is falling".
sorry, i'm not suffering from anything. but for you would be good when asking for advise to give specifics. in your case, it's VA "i.e. US government policy" and there is a VA in Thailand you can register with... so what you looking for is not regular insurance, but insurance through the VA in Thailand "which one could not reasonably assume from your post". however if you go to a regular insurance company, pre-existing conditions would definitely apply, they don't care if you are a veteran or not. So unclear why you'd even bring up pre-existing conditions to begin with. Also I know people who have VA insurance in Thailand, and I'm certain VA only covers Medical conditions from when in the military, and not certain they're going to cover diabetes for you, however fell free to have your own delusions about that... i.e. others I know have the VA there, and private insurance for other issues unrelated to their military service.
whats to explain. they will insist to get your medical records from your home country, and then any pre-existing conditions will be called out on your policy that that's not covered. even in US except for company "group" policies and medicare is exactly the same. Insurance companies, to the surprise of many is a "for profit" enterprise.
Local flights within the country are one thing. International flights I'm not sure there's any way to do in cabin. I was directly behind a woman with a very small dog in the approved carrier upon boarding and flight attendant told her to return to the boarding gate, wasn't interested at all about paperwork. Never saw her board the flight afterwards. I fly International a lot, and have never seen a pet on board so far