Hey Bigd, I entered Thailand this year on an OA visa and like you, I had many questions. I went to Bangkok immigration to learn about the renewal process and can confirm a few things written here, at least according to them.
Yes, you can extend the OA within the country. And contrary to what some people on this board say, IT IS an immigration requirement that you complete your 90 day reporting. Perhaps there are some other provinces where they don’t look at this as much, but certainly in Bangkok they made a point of telling me that it is a legal requirement, if you are in Thailand for 90 consecutive days. If you go traveling outside of Thailand before the 90 days are concluded, then the clock on the next 90-day reporting starts the minute you return from your travel, and you must complete your reporting at the next 90-day interval. Also note, that the first time you do 90-day reporting, you must do it in person at Thai immigration. After that, you can do it online. 
Also, I confirmed that I must change my excellent international insurance to one of the approved Thai insurances at the one year extension or they won’t approve me. You’re right, that it is not clear on the Embassy websites and in fact the Thai Embassy in my country told me she believed I would be able to continue on my international insurance at the first year extension if they filled out the foreign certificate again. However, she told me to double check at Bangkok immigration, which is what I did. They told me in no uncertain terms that the rep at the Thai Embassy was incorrect. I must choose one of the approved Thai insurances to extend the OA, OR I have the option of leaving to a neighboring country and starting the process over for the non-immigrant O, which does not carry a health insurance requirement.
There are different options regarding the financial requirements, depending on what your long-term plans are. Since I don’t wanna go home every two years to reapply, which seems unnecessary, my plan is to deposit 800 K into a Thai bank and just let it stay there. I understand this is not everyone’s choice, but it will allow me the most latitude whether I extend the OA or begin the process again with the O. So ultimately you have to decide. I hope this helps.
As you can see a lot of people have different experiences, but these boards have been very helpful to me. However, I encourage you to go to Bangkok immigration and ask questions there, as I did, because some of the opinions on this board are just not accurate from the current legal policy, and if you’re doing this yourself, as I am, it’s better to do it with information directly from Thai immigration, especially on points where you can see some people disagree. Also, be aware that rules seem to change frequently, which is why it helps to do things directly at immigration, even if it’s a bit of a pain in the ass🙏 We are foreign guests here, and it helps to learn the system, even if it frequently changes.