TL;DR, you need a magnetosphere to keep the solar wind from stripping your atmosphere. You could create an artificial magnetosphere by placing a really powerful magnet at the first Lagrange point such that the planet is shadowed by the object’s magnetotail.

This is a lot simpler than what I had conceived, rings of satellites orbiting the planet and doing… something that keeps the atmosphere stable. Of course there’s a lot more to maintaining a terraformed climate than just the magnetosphere, so maybe I’ll need those satellites after all.