Both thematically and mechanically, I love the idea of introducing orbital solar reflectors. They should be a ship module that's very large (500k tons each?) but relatively low cost, and you need lots and lots of them. That way they have to be built in-situ and can't be moved, but aren't prohibitively expensive. And they are vulnerable, much like orbital habitats, so you have to defend them more actively than just stationing ground troops.
Have them cause a % reduction in body max or min temp, whichever is furthest from the species nominal? That would allow them to be used for cooling, heating, or temp range reduction as needed for that body, all automatically. That way they have to be built in orbit of that body, and can't economically be moved, but aren't prohibitively expensive. I'd think duranium, probably nothing else. If you wanted to be super scientific we could weigh the effect by body distance from the star and star luminosity, much like terraforming is weighted by planet diameter, but that's more complicated to implement.
They should definitely be locked behind a low-ish cost tech. Could also have a tech to reduce material/cost to build, but that seems optional from a gameplay perspective.