I am in the same position as you, I am doing the none o retirement visa, using the banked 800,000 as my emergency medical fund. If you leave the 800,000 in a separate deposit account, you can get 1.5% interest, so the fund is not totally wasted. The cost of insurance with pre existing conditions is horrendous, and gets worse as you get older.
It’s a lot cheaper if you get the initial 90 none O visa before you come. Also depends if you want them to front you the 800,000. My friend has just had a price of 28,000 baht, for a multi entry visa.
I did not need to show my TDAC today when I came in on a none O visa. The IO said that it was linked to your visa, if you put the visa number on the TDAC form. I was questioned about what country GBR was thought.