It's already been pointed out how easy it is to destroy an opponent by camping and blasting their Starports and this is worse than in GC2.
I would like to see the planets retain some level of ship building capability, say only Tiny or Small hull types (and maybe medium at a higher cost.) Build the larger ships in space, but you'd still be able to build smaller ships (and constructors) on the planet.
The ability to only construct a single starship at a time at the starport/planet is/was a little lame. Consider adding the ability to have multiple build channels (kind of like naval shipyard slipways) and allow multiple smaller ships to be constructed at the same time (both planet and starport) with larger hulls requiring more of the "build channels" so that a starport might only be able to build a single capital ship at a time. Perhaps allow constructors to add/enhance build channels to a starport. You could then build a seriously large shipyard in a secure area to pump out ships.
An alternative way to accomplish this would be to allow a single planet to service multiple starports. Then I load up the planet with manufacturing capability and surround it with starports.
Probably too late at this point, but I think the Starport/shipbuilding capability could have been just a starbase option/module. This would allow adding weapons and defense to the ship building capability.
I'm not seeing a way to get rid of a starport that I don't want and the game insists that I attach it to a planet before I can continue the turn.