You can fade the sound to the rear to bring them up more.
If you replace the rear speakers you probably won't make them louder. If you stay with the 2 ohm impedance the aftermarket speakers will probably be more power hungry than the OEM ones so will not be louder. Sound better, yes. Play louder, probably not.
If you replace all 4 door speakers then you might bring up the sound.
The door speakers are mid-bass speakers. Adding 2- or 3-way speakers will change the sound stage as the OEM system just pumps highs out of the dash and bounces them off the windshield to project it through the cab.
Some combinations will be better than others and what one person likes another will not so you will have to make your decision based on what you read and what you like.
Best of luck and post your journey for the rest of us to learn from.