Assets worked on in blender and saved in .blend files in the ignored ".raw blend" directory.

Assets exported to .glb files. Then imported with internal textures (this is an import setting, use "Embed as basis universial").

Materials created in Godot can be applied to imported assets. Make a new inherieted scene from the imported asset. From there materials can be loaded and modified.

In Godot materials can be saved by clicking the down arrow on the material selection box and picking "Save". This will save the material as a new material with the current settings.