There are already visa’s for agricultural workers. The rules & regs for these visa’s need fine tuning to make them work better for Ag employers to use to get workers in, but there is no need to give amnesty based on the agricultural industry.
They should be given 6 months to leave the country and then apply for the working visa once back in their own country like legal immigrants have to. Once there they then are vetted and have to pay a fine. Not all will be granted visa’s , but that is like legal immigrants who never know if they get their visa.
If they do not leave after 6 months then they are arrested at their places or work, welfare is cut off, and the employers fined which can help to pay for a fence..