Could we control the weather with a disk in space?
Maybe this question belongs also into the space department here.

The only Lagrange point that makes sense is Earth - Sun - L1, and it is unstable (needs continuous correction). Satellites have already been put there, e.g. SOHO. But what we really need to do is cut down greenhouse gas emissions, anything else is not sustainable, or not practicable (a 300km sail in L1 belongs to the latter). Looking at the problems NASA has with the heat shield of the James Webb telescope i am tempted to say that it is not makeable with current technology.

Geostationary orbit is not an option. The spacing between satellites is much too small and the shield would only cover the equator. Even in geosynchronuous orbits the satellite spacing is down to 73km (https://en.wikipedia.org/wiki/List_of_satellites_in_geosynchronous_orbit) i read, so that's also occupied for such a frond. It'll collide in the ascending/descending nodes with other stuff.

The risks of geo- and climate engineering, e.g. lack of knowledge about possible side effects and the potentially drastical acceleration of global warming when engineering efforts cease (out of money error ;-)) and the time when they were in effect has not been used to cut down greenhouse gas emissions, are huge.