Advice on workflow.

I'm currently in the process of designing a Nerf Blaster from scratch and keep running into issues in my design process. I have the concept designs finished but struggle trying to find what to do first and how to get everything to fit smooth. I think I've got it then realise I need to edit components I thought were finished to fit in other components.

I'm going to wait until I've got all the internals and do a test rig to make sure everything works then work out where everything is going to fit and where to put wiring channels.

The shell is going to be 3D printed so I also want it to be in pieces that are as easy to print as possible.

I'll be using thread inserts to bolt parts together.

So far, I'm trying to start with the shell, with external details and then the internals parts. Am I better working on the internal and smaller parts and build a shell round them?

Any help or advice would be appreciated.