Wow, indeed correct ! Never seen nor heard of it. Well that makes it ever harder in the future for you mods to find out what visa someone has 🤨, pff. 'Chokdee' I would say.
and you say per fortnight $840 abroad. So about 40,000฿ a month. And yes, you can already live with that in Thailand. Paying per fortnight is a good idea I must say. But....getting totally off topic. Sry.
In Korat immigration they already 4 times tear out the old one and put a new one in counting 90 days from the date I arranged the extension. So that's nice over there.
And don't forget to ask a re-entry permit if you want to leave and come back to Thailand within that year extension !! A year extension is NOT multi entry as your visa is now (for 90 days)
From what I understand is the only exception for mandatory health insurance for extension of stay, when y'r original visa (mostly the visa sticker in y'r passport) is/was a non imm O-A visum.