Is there any better way to make this transparent dome?
The filament seems to lose its translucency past a certain thickness


That's exactly the point. Think of translucency as a percentage of light being allowed through, per amount of material. 2x the material means 1/2 the light. You need either less material (which becomes brittle as you saw), or a more translucent material. I think it has little to do with the model itself.

Also - check out OnShape. One of the features it has is a "shell" function, that does what you're describing - takes a single 3D feature and creates a shell out of it of consistent thickness.