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I guess if I were you I would consider which job you like better- sales and estimating, or operations management. Once you have decided start looking for someone to do the one you like less.
It seems to me that you are doing some pretty complicated jobs and cranking them out. It's going to be pretty hard to replace yourself there. Good luck even finding someone competent. If you can find a good salesman tie his commision to the NET profits. If you give him a percentage of the gross he will lowball all over town to build his own income. It doesn't matter how much you pay the top guys, as long as they are making you enough to justify their pay.