I rerally did not have a problem with the Agency I used. They did all the leg work - back a forth to different offices, translated all the peperwork - including the Thai Marriage Cert post marriage for the British Embassy and, if she comes to the UK, everywhere that wants a copy of the marriage cert (immigration for example). It cost be Baht 5K ten years ago, so am impressed its only 6k now. I'm not sure why you say you haven't met an agency you trust, because what's there to trust - you pays your money and they does the job (actually the other way around) - several people I know used them, and none of them had a problem (some variance in price though). If you are happy to do all the leg work, then go for it - I preferred to sit in their office sipping coffee-yen with my Mrs to be - worth £100 all told to me.

By the way, unless it changed a lot, the Amphur is just sitting at a desk signing ten ton of forms. There was a queue when I went, though smallish, because of all the forms, it was slow. My agent gave someone the nod and they opened a desk for us. As they drove, it also saved on the taxi fare.