A better alternative to a sunshade at L1 is a swarm of autonomous hot air balloons, which can change their reflectivity. A nice thing about this is that they do not need to be built all at once for the whole earth, but rich cities in north can use them to get more sunny days, and places like dubai would use them as artificial clouds.
But they are much cheaper, can be useful even when partially deployed, and allow fine grained control over weather instead of making everything uniformly colder.
