Serum free light chain test shows normal kappa (8.7mg/dl), low lambda (3.80), high kap/lam ratio (2.31). Serum $ urine electrophoresis show no monoclonal protein, nor does serum immunofixation. Likely MGUS, myeloma, or admyloidosis? CBC/met panel ok.

False positive rate. There is 30% or so false positive rate of kappa/lambda ratio in people without MGUS, myeloma or other disorders. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex.
None of the above. With no monoclonal protein in serum or urine, all of these are unlikely. I would not be concerned in the least about being a guy who just happens to make more kappa than lambda antibodies. You know that reference ranges are set so that maybe 5% of healthies fall outside. You're 31. Get / stay fit, find / share love, work hard career-building, and enjoy being young. Best wishes.