Galactic Power Value per credit - mods vs character levels?

You're right in that completing combat missions will be a huge source of GP for the territory. I'm guessing your Rey is closer to 15k, not 30k, however, but that just illustrates the point even more.
It's important to remember, however, that having a lot of 7* LS will certainly help with platoon missions, which can be vital to actually being successful in later waves of those combat missions. Gear and level seem to be far less important, though.