Well, if you are looking for more drones, you could have the logistic upgrades provide an additional drone per level...as of now, only the first upgrade grants an extra drone (so most planets have only 2 drones)...
In all honesty, even if you doubled the HP of a drone, mid-game fleets or even 2 carrier caps outfitted with fighters are going to wipe it out pretty quick, so unless you get to outrageous numbers, a simple HP/armor buff to drones isn't really going to help...
You could make it so that drones respawn every 30s even if enemy ships are present...at least then the enemy would have to continually refocus every 30s to prevent you from building structures...not sure how I feel about that solution though....
I hoping with Rebellion, a research modifier will be added that allows techs to grant extra constructors at each planet...