You are missing the point and failing to address the example I provided. Since your only avenue of expansion is starport and if starports are basically defenseless what happens when it gets destroyed by a single enemy fighter attack in the very first turns of the game and you get camped? At that point it doesn't matter how cheap starports are because every new starport you build will immediately be destroyed by a simple enemy ship. The enemy will be able to destroy all your new starports with impunity and expand while preventing you from expansion which means you lose. This is why what you propose is a terrible idea. Because it reduced an otherwise grand strategy game that would last for thousands of turns into a simple one of early game rush.
Let us address the initial rush of the game and the problem of the rush in the beginning of the game. At what turn would you deem it plausible that one can destroy an enemy starport? In a tiny game, I was able to destroy the enemy in 3 turns, for Gal Civ 3.
Now, in Gal Civ 2, which doesn't have starports. I did (I played it out just now to see) effectively blockaded and dominated the other opponent at turn 10. By that turn I was pushing out a combat ship per turn and eventually when I got planetary invasion I couldn't do much to counteract my invasion. Having 15 ships around each of their planets effectively destroyed them. The only reason it was turn 10 was because i didn't have military ships to start with and had to research the tech. For note my main planet had nothing but research centers and my other planets had nothing but research centers. By the time I started researching planet invasion, it took me 30 turns to get that. But it was inevitable that I won since they couldn't keep up with the number of ships I had compared to them and the speed at which I can research.
I did address your issue, and the issue is not the fact that starports are too easy to destroy. Give the starport 1000 attack for each of their beams, kinetic, missles with 1000 defense. I would still win the game within 3 turn (unless their starport can attack) because I would be able to destroy every ship that comes out of that starport. Because I could have 15 ships surrounding it by turn 15. Each turn there after I could let myself build up what I will to win the game. It is not the fact that the starport is too easy to destroy, it is the fact that Drengin start with weapons and can start too close to the enemy to dominate them militarily.
Civ 5, does not have this problem, why? The cities are difficult to take yes, but more importantly EVERY empire can build weapon bearing troops at the start of the game. I've played against the tiny game rush, guess what I couldn't win because I was effectively cut off from building settlers (I didn't expect my enemy to rush warriors to destroy my initial build off)... I would have to build troops to counter them, which I did (too late though) and they effectively matched my troops, soldier for soldier. They had a better economy than I did since they were able to expand more than I was able too. Having troops stationed on a tile prevents me from getting the resources of that tile. But I didn't feel helpless in the game because I was able to build weapon bearing troops from the start to fight against that onslaught. Even though I knew I lost the game. Solution, play a bigger map or start building more defensive troops at the beginning of the game.
I would like to see prototype weapons at the start of the game for each race. Weapons that have horrible accuracy, can overheat, overload and not fire. The research is to make these weapons more stable and consistent. Then the Drengin starting with effective weaponry doesn't make them dominate as easily in the rush game as they would have to counter act these prototype weapons. Sure they can do so, but I can now defend myself from that onslaught. When I see it coming then I can research the weaponry to defend myself better and match the Drengin. It would also make other players wary about invading to doing the cheap tactic of destroying the starport early game.
Another option since we don't know how tactical battles will take place, we could force defending scout ships (tiny or small ships only or possibly larger ships) the defensive option of Kamikaze. Giving them the ability to destroy attacking ships at the loss of them. The defense against this tactic is kinetic and they do as much damage as their HP or half their HP. But only as an option at the beginning of defensive combat, so you can either choose to use this tactic at the beginning or not at all.