I understand that your bottom is open for air everywhere?

This seems too much IMHO. I would close large areas maybe simply with duct tape so you can change/improve it later. The air should enter right where it is needed: directly under the HDDs, and right next to that big blue block (GPU?), and maybe right under the PSU if it is located close to the bottom, too.

If you don’t restrict the air flow a little, then all the air will take only the easiest way, that is through the open space, and not between the hot devices.