Can you work on the farm? I don't know what "farm work" technically is. IF you want to work legally, you'll need a work permit. You can get a work permit on a Non-O Visa based on having a Thai wife. See
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for setting up a business, and the forbidden jobs
But employment law isn't visa related, and if you or are your wife wants to start a business together, I recommend contacting a good, high quality lawyer. Your embassy may have a list of lawyers they can recommend.