Stratospheric Aerosol Injection [1] for emergency cooling sounds relatively cheap too, and has the advantage of a) mimicking a well-studied natural process and b) being inherently self-limiting. Your "if things start getting chilly" fallback may not be viable if e.g. a Kessler cascade locks us out of orbit.

[1] https://en.wikipedia.org/wiki/Stratospheric_aerosol_injectio...