That big hole is the inside of the shield generator ring, if you can see. *Insert some lore friendly excuse why the shield generator needs empty space on its inside to channel magnetic stresses from the fortress shield ability.*
Indeed. Those magnetic stresses are exactly the reason you stick half the bridge out in the middle of it. Intense magnetic fields are proven to increase bloodflow and cognitive ability in captains. I always thought it was there as a safe dock for fightercraft.
Anyway, if it looks cools and it feels cool, it's probably cool.
Speaking of...
I've found that it's a lot easier to tell when I'm done shading by looking at the shader layer by itself instead of looking at the resulting layer. If I can get the detailed shape of the ship from the shader layer, it's probably done. If it's indistinct, I need to do more.
E.g
Spoiler
as opposed to,
Are there any other good ways to tell when you're done shading?
Edit: My brain and my hands had different ideas when I wrote that first sentence.