Concentrated solar thermal gets much better performance than nuclear thermal because you're only limited by the materials properties of your heat exchanger, not your reactor fuel. The problem with solar thermal is three fold.
First, you have to be able to see the sun, so your burns are restricted to times where you're not in Earth's shadow. That could make transfer windows difficult to hit, of course if you have a thousand seconds or higher ISP, that's not such an issue.
Second, we're talking about high thrust rockets so you don't have astronauts sitting in the radiation belts for days as a low thrust rocket spirals out. That means you're going to have to have significant structure to support that concentrator, and it's no longer going to be very lightweight. Of course with such a high ISP, this may no longer be such an issue.
Third, this reflector cannot simply be static. It needs to be able to articulate to whatever angle you need to thrust, which greatly increases the complexity and mass of such a system.