May not be many choices sadly perhaps just see if there are any specialists you can communicate with. I would imagine there would be a list of some sort at the hospital itself. I wish you luck having had Crohns myself.
Countries that have the low numbers have them because of the steps taken by their leaders and the self discipline of the people living there. The opposite applies to those countries with high numbers.
For retirement Visa Thai Consulate Sydney said if you can get health insurance for the right amount of coverage you can use any health insurance company but that was just before the border closed. Probably easier to use one of the listed ones and maybe look elsewhere after the first year.
Suggest you contact Thai Embassy and check requirements there. Obviously at the moment no foreign people are being admitted to Thailand at all until further official notice.