Why do planets require solar mirrors to terraform even if the termperature is already ideal?
Shades/Mirrors can be used for far more than just cooling/warming planets as a whole. They can change environments entirely.
Consider the Sahara Desert. It might seem completely barren now, but there's lots of evidence to suggest that it used to be a lush savanna barely 10,000 years ago. This is because of this thing called "Milankovitch cycles." Axial precession of the Earth caused more solar radiation to reach the Sahara, heating it to a point where the air above it became significantly hotter and less dense than the air above the Atlantic Ocean, causing the dense and humid ocean air to regularly flow over the desert and shower it with rain until it was a desert no longer.
You could theoretically use solar mirrors to achieve the same effect.
Now, seeing as Starsector's water worlds are canonically so deep they make the Mariana Trench look tiny, I'm not certain by what mechanism they could be turned Terran, but I'm pretty sure the mirrors are a better tool than nothing.
Which reminds me... if you wanted to add more realism, then changing atmospheric density should affect planet temperature. Thin atmosphere = colder, thick atmosphere = hotter.
I don't think the link between atmosphere and temperature is that direct. Sure, you have Venus, which has an atmosphere that makes its surface hotter than Mercury's despite its greater distance from the Sun, but you also have Titan, which has an atmosphere 1.5x denser than Earth's, but isn't all that much warmer than the other moons of Saturn.