Hi!
Gotta express joy of seeing a detailed, specific question with screenshots. That is really nice.
Your performance is totally on par with what would I expected from this scene. Your guess about you being render thread bound is totally correct. Your number of draw calls is at least two times higher than what I would consider “maximum acceptable” for myself on a PC scene. I would not expect CPU usage to reach 100% before you start experiencing highly noticeable bottleneck.
Technically, you should try merging your the objects in a single mesh or try using instanced meshes. It should drastically improve performance in your case.
GPU clock speed and load are also not the most conclusive factors. You should consider giving GPU profiler a try.
Lastly, ensure that you are doing tests in standalone game. Play in Editor usually has added costs.