Blender: Clean GLB Export for Godot

I have created a map in Blender 5.2 that needs to be exported as a GLB file for use in Godot 4.7.1.

The map consists of:
Coloured surfaces representing the individual states and the sea
A continuous black outline separating the individual regions
I am currently experiencing problems with the black outline. It was created from a plane mesh consisting only of edges aligned along the X and Y axes.

The following approaches have not produced satisfactory results:
Converting the mesh to a curve causes inconsistent line thickness.
Converting it to Grease Pencil and subsequently back to a mesh also results in uneven line thickness.
The object scale is correct, and there are no duplicate vertices or edges.
The outline is not obscured by the coloured surfaces.
The Wireframe modifier is not suitable because the outline consists only of edges and has no faces.

I have already tried troubleshooting this issue extensively with both ChatGPT and Claude, but neither was able to provide a satisfactory solution.

Required deliverables:
A clean, exportable GLB object with smooth and consistently thick black outlines.
The complete map and all related objects prepared at an appropriate resolution and scale so that the outlines and objects are displayed correctly in Godot without distortion.
The individual states, the black outline/frame, and the sea must remain separate objects in the Blender file and the exported GLB, matching the structure of the files provided.
The final GLB file must have an appropriate and optimised file size for use in a game. Excessively high polygon counts, unnecessary geometry, and unnecessarily large textures should be avoided without compromising visual quality.
The complete editable Blender source file (.blend) must also be provided.
A short explanation of the method used, including the relevant Blender settings and export steps, so that I can understand and reproduce the solution.

I have already spent considerably more than a day trying to solve this problem without achieving a satisfactory result. As I am still relatively new to Blender, I am looking for someone with solid experience in Blender-to-Godot workflows and GLB exports.

When submitting your proposal, please include:
A brief description of your relevant experience
Your fixed price or estimated cost
The date by which I can expect the completed GLB and Blender files


Kind regards,
Dominik
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