Test #3: Overexposed
Test #4: Underexposed
Test #5: Underexposed
Test #6: Lighting is about right. Does this software have f-stop/depth of field? (Would incorporate a nice touch) Also zoom in a bit more on the build as seeing the sides is a bit tacky if you know what I mean. :/
Test #7: Almost there
Overall, I think that this is a great build, however being a CG artist myself I do see many ways you could improve on the renders. If you ever need help with renders, just let me know! I'm always there to help.