Warning64 I know a thing or two about dreadnaught building as well, I'm not as talented as @The_Bloodnaught but I should be able to tell you a couple useful tricks.
Use modules! I see you having a walkway already across the ship, that is very inefficiënt. In a module crew will do what needs doing, and won't walk across the entire ship.
Design modules before you start adding them to your ship. I see you are building a lot step by step, but it is much easier to design modules and paste them where they need to be. You can make many different modules for different purposes and places.
Broadside railguns are not that good, if you use them brotect them at least well enough with multiple shield layers.
Keep the paint simple, as it is difficult to make it look good and detailed at the same time on such a scale.
If you care less about the paint, don't fill the ship with armor or corridors and just keep the space inside the ship empty for most part.
That should do it for the basics of dreadnaught-building.