I like the idea to design my star bases in the same way as ships.
There should be a special hull class "Starbase".
The difference to ship design is:
1. No drive can added.
2. The starbase costs is calculated in 'Number of Constructors'
3. Any starbase has special slots for functional updates (as support or bonus modules) which can be added by separate constructors after the starbase is build up.
4. A starbase design can no6t be selected in the ship build screen.

If you command a constructor to create a starbase, you have to select an already created starbase design. Then you need to add as many constructors as the design uses before the starbase is ready.
Starbases can be updates in the same way as ships, but I have to pay for the update by sending additional constructors.

An additional possibility is that every planet can build his own defense starbase in orbit, simply by selecting a starbase design in the ship build page. This starbase need no constructors to be build, it is payed by money as any normal ship, but it can not hold any functional updates - only weapons, defense and sensors. (Similar as the starbases in MOO3).

One logical advantages of this idea is, you can not create starbases with weapons or defense technologies you don't have researched. As a practical advantage you have more possibilities to adapt your starbase layout to your strategical needs.