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[inquisitor]

From my understanding the STK application on toggle SIMs works completely passively as there's no method to actively scan for networks in the STK standard and so it can't really drain your battery.
Instead the process works as follows: As soon as your phone loses its signal and tries to reselect another network it will submit the MCC/MNC-tuple of the found network to the SIM as part of the authentication process. Now if the MCC is from a country the SIM has another IMSI from it simply switches IMSIs. So it's a completely passive process.
I would rather assume that the obviously buggy STK implementation of your handset is responsible for the battery drain.
Have you actually checked for a firmware update for your phone?