@Damo *******
Honestly, if I were you, I would mentally prepare myself for the extension of stay based on STV to be rejected. I was helping someone today who entered on a STV with 3 months insurance, got another extension for another 3 months, and Bangkok Immigration rejected her with the extension.
You'll also have to meet the same requirements as you used to enter on the STV for the insurance and pre-booked hotel (or a condo that you owned). I know in Bangkok they are being very strict about issuing those extension of stays.