I am also flying on Wednesday, going to do mine tomorrow afternoon. As I don’t land until 13.25 on Thursday local time. Which is where the 72 hours is counted from.
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.