It's a couple of years since my last visa extension, also in Chiang Mai, it usually took about an hour, there used to be place on the opposite side of the road where you'd be directed for additional photocopies etc. All very easy to do, no need for an agent.
I'm in the UK but for a 60day tourist visa the embassy wanted to see minimum £1k in my account and for the special tourist visa (90days and renewable) it was £5k.
It's my understanding that you'll have the same requirements in 3months when you're planning to renew, insurance and accommodation paid for the following 3months so its not ideal. Btw I've opted for the STV too, going this weekend.