the documents need to be updated, not the ones you previously showed plus you need to show the 500k baht in the bank. It's up to you which route you choose but note you can only extend once then you have to leave anyway
not sure you are understanding. You won't be able to do your extension yourself (its a new extension every time, never a renewal). You will either need to continue using an agent or leave Thailand and start over with a new 90 day Non-O visa
that would be the case if you'd done your current extension yourself by meeting the requirements of it but you didn't. If you were to go for your next extension yourself they will also check that you have met the requirements of your current extension, which you haven't so they will refuse the next extension. So when this extension is running out you only have 2 choices, either keep using the agent or leave the country get a new Non-O and start over. My question about which Bank you got the account opened in is also relevant as I suspect agents can no longer fiddle things via Bangkok Bank
as you have used a lawyer as an agent to open your bank account and obtain your extension of stay you are going to have to use an agent next year also, you won't be able to do it yourself as you haven't met the terms of your current extension. Also if your bank account is with Bangkok Bank it will quite possibly be at risk of being frozen unless you go in and verify it yourself with passport, thai phone in your name and receipt for the sim
If you go for an extension then you have to provide updated documents for for everything including the reason you obtained the DTV, additionally some offices want you to show you had 500k baht for a month before you apply. A border bounce is much simpler