Unfortunately, the level of this blog (and not only this one) is really low. Those who are looking for an explanation for their problems can be amateurs, but those who give answers and explanations must be at least semi-professionals.
I have a 3-phase solar system with iSolar SMP 5 kW...
Sorry, I missed your question. The MPPT charger in the SMG is not the problem. So using an external MPPT charger does not solve the shortcomings of the inverter itself.
I did a little searching to find out the difference between SMG II 6.2 and SMG III 6.2 but it is not possible to find out from the data provided by the manufacturer. In my opinion, there are no significant differences. As I already wrote, the problem is the data that the manufacturer does not...