As Sam2 says, just say it silently by mincha. That’s what I do as well. I’m in a similar situation, davening a nusach that is in between Ashkenaz and Sfard but closer to Sfard on these things, while virtually all minyanim here are Ashkenaz.
In any case, the idea of “davening nusach s’fard for mincha and nusach ashkenaz for ma’ariv” is absolutely not possible. One must daven either one nusach or the other. That said, if you are shatz you use the nusach of the shul – even if it’s not your nusach, but for your own private shmoneh esrei you use your own nusach. That’s how I’ve always done it.