If someone is elderly, it should be evident that this insurance requirement will ultimately lead to a dead end to their stay in Thailand. If they intend for a "long-stay" here. Eventually, they will reach an age where the policy is no longer renewable.
I think you mean 800K bht not $ dollars .Maybe a workable plan. Arranging for the deposit of 65K bht each month from a foreign bank, that satisfies the scrutiny of Thai immigration, could be a challenge.
I think there will be issues with insurance policies being in sync with visa extension time frames. I think some (if not most) companies will not sell policies by the month ie they only sell policies for full year coverage,