As far as I’m aware, the rule of thumb is whatever you say quietly you do in your own nusach and anything out loud ie. kedusha etc you say in the Shul’s nusach.
That is my rule of thumb as well. I daven Ashkenaz, but the only time I’ll say something in Nusach Sefard is:
A. If it’s something the tzibbur says as a whole (i.e. kedusah, the second barchu in Ma’ariv, etc.)
B. If I’m the shliach tzibbur (I won’t daven for the amud in N. Sefard, but I will lain in a shul that does, and will say the kaddish after the kriah).
The Wolf