I've never had a problem getting annual extensions. The original visa was much more work than the extensions in Thailand.
I would say to avoid getting an O-A visa so you don't have to worry about the extra insurance requirement. There are thread on the forum here about how to apply for an O visa after you arrive in Thailand. The hardest part with that seems to be opening a Thai bank account which you will need to get extensions. If you already have a Thai bank account great.
I also find it easier to just leave 800,000 baht in a separate account to use just for immigration.
Also you do need to think about medical insurance here. If you don't have any pre existing conditions right now it would be best to have insurance already. All the insurance companies here will try to exclude everything they can. It also gets quite more expensive the older you get.