Unfortunately, so far there have not been any other Cinema 4d users in this forum, nor do I know any users within High Fidelity that any longer use it.
FBX importing should be straight forward, and has previously worked. However last time was probably @Judas back in 14.
I have not experimented with substance materials into fbx (as blender fbx does not natively support that).
Instead export the textures building the material in Substance painter, exporting roughness, albeido, normal, metal maps, then applying them to the models in Cinema 4d and just testing it out. (sorta like what I did with blender ages ago, which most of the community now uses). Unless C4D has PBR support natively (which I dont think currently is supported, until @sam would get example fbx), I suggest mapping it the following way:
- Normal - Normals
- Specular Color - Metal
- Color - Albeido
- Hardness / Shinyness (which ever) - Roughness
Atleast thats how it maps for both Maya, 3dsmax ( until both got stingray) and Blender: the ancient reference here for previous can be found here
GLTF on the other hand is a bit incomplete, and is still being worked on. What were the issues you were having with the gltf import?