Not to sound like an LLM, but that seems exactly right to me. Use it as a cheaper, high-functioning task subagent and lower reasoning for a master Opus session. As long as not every portion of your task requires maximum intelligence, you should come out ahead.
Won't any input be charged uncached, and the output of the small model charged again as uncached input to the bigger model?
I don't know whether that comes out ahead compared to just staying with the better model in the first place.