How can I add more insulation to this roof?
Spray foam insulation is one of the best options for insulating.  It seals very airtight (losses from leaking air can be 40% of your energy bill) and can cover over metal and other materials that can "thermal bridge" and conduct heat from outside.  It is expensive, and is generally applied by a professional which may put it out of your budget.  It is also somewhat flammable and some areas require that it be covered by an ignition resistant material which might be difficult.

Next up would be solid foam panels tightly fit between the trusses.  Third best would be more fiberglass batts like the ones up there already for a greater thickness.  You are trying to increase the air tightness and the total R value.