I converted your GLTF file into GLB though GLTF-Transform CLI with the next command:
gltf-transform metalrough fefco.gltf fef2.glb
Here is the result in the Sandbox.
The original (without conversions from spec/gloss to metal/rough) looks more than very much similar.
There is also some z-fighting in the right upper corner.
I believe you may completely drop off the textures and change all GLTF materials to the new PBR materials with the needed metal/roughtness values (if the materials are always the same, there is no need to use extra materials with each model).