AFAIK SMT is like a MLU for older Mig 29 users. Its the appropriate choice for older Mig29 owners wanting an upgrade. Its not offered for newer customers as the same capabilities n abit more is being offered with the newer Mig 29M,M2,K, and Mig 35. Why would a new customer choose for a new build SMT when the logical choice would be a new build Mig 29M/Mig35.
And regarding Airframes, the Mig 29M,M2,K, and Mig 35 shares much closer airframe design/parts compared to the older Mig 29, which could explain how Mikoyan is building the Mig 29K for India. This is the reason why there is a SMT upgrade for older planes, as they cant be upgraded to Mig 29 M,M2, Mig 35 airframe.
And the Mig 35 demonstrator is currently using the Mig 29M2 demonstrator airframe. the final build may have slight differences in having more composites, and some small redesigns in the vertical tail and spinal tail section.