I've done my lady three years in a row. Last one took five days. Never used an agent, as there's nothing they can do except fill in the online form for you. Only mugs would pay that much money for an easy task! You've been duped, but you're not the first and you won't be the last!
I've done 3 x UK visas, 2 x Australia and 2 x Schengen for my lady. Never a problem. Most important thing, letter from employer showing position, length of service and salary. Next up bank statement showing good savings record. Thirdly any land/property holdings. They want to see reason for returning to Thailand. It gets easier the more visas they've already had.