You're right. The amount of thrust available for graviational slingshot maneuvers is insufficient; it's insignificant compared to the solar power advantage of Venus.

However, it's still possible to play gravity games to usefully steal energy from Martian moons, using a more direct approach. Instead of trying to get a graviational boost from a Martian moon, you can manufacture an aeroscoop tank factory on Phobos.

The aeroscoop hardware is a small reusable module which lands on Phobos and is latched onto the aeroscoop tank.

Then, the aeroscoop ship lofts into an elliptical orbit that scoops up upper Martian atmosphere while de facto aero-braking.

This aero-braking lowers the orbit with each pass, until the tank is full and the aeroscoop ship circularizes its orbit.

The aeroscoop ship docks with an orbital habitat in low Mars orbit; the small reusable module detaches and returns to Phobos.

Basically, the mass of Phobos is lowered into a lower orbit in the form of storage tanks in order to lift Martian gas into orbit.