About 2 years ago at CW, I asked what happens if one is too ill to go in person for an extension of stay based on retirement. I was told by the IO that someone could represent me with some kind of medical document. No specifics were mentioned, but the IO probably meant a Bai Rap Rong Pat ใบรับรองแพทย์ (medical certificate)), but my status would change to something like "Sick", and if, for example, I happened to be grandfathered (in Thailand continuously since before Oct 1998), I would lose that status and if I recovered, I would have to start all over again under current rules as if I had entered the country for the first time. That would mean I would have to bring in B65K/month or put 800K in the bank, instead of 20K/month or 200K in the bank under the grandfather clause. I did not ask for any more specifics at the time and the above is all I remember. It is also possible that the medical certificate must be a certified copy issued from a government hospital. My only advice would be to ask your local Immigration Office before you get too sick to report in person. Item 25 on this page has more information:
https://bangkok.immigration.go.th/en/visa-extension/#1610937669254-c6462860-6f05
25. Visa Extension - In the case of receiving medical treatment, attending rehabilitation, or taking care of a patient
In the case of receiving medical treatment, attending rehabilitation, or taking care of a patient:
Criteria for Consideration
Must have been confirmed and requested by a physician of the hospital providing said treatment with full explanation about the illness, period of treatment, and the physician’s opinion that the illness is an impediment to travel.
In the case of taking care of a patient, the applicant must have been confirmed and requested by a physician of the hospital providing the examination and treatment or by an embassy or consulate.
In the case of caretakers who are not the parents, spouse, children, adopted children, or spouse’s children, permission shall be granted for no more than one person.
Documents to be submitted
Application form
Copy of applicant’s passport
Letter of confirmation and request for a temporary stay issued by a physician of the hospital providing said treatment
In the case of taking care of a patient, the applicant must attach a letter of confirmation and request for a temporary stay issued by a physician of the hospital providing said treatment and documents stating relationship (in the case that the caretaker is a member of the family), such as a marriage certificate, a birth certificate, registration of child legitimization, child adoption registration certificate, or confirmation letter from the embassy or consulate.