I agree with what others have said, you really need more opening from the FB to the CC.
At least one more pipe, and get rid of the damper in the pipe between the firebox and cook chamber.
Better yet would be to cut it off, get rid of the pipe and attach the firebox directly to the cook chamber.
Also appears that your firebox is too small. It should be at least 1/3 the volume of your cook chamber.
Definitely want to add more air intakes to the firebox. They should also be below the level of the fire grate, so air feed the fire from underneath.
Not sure why you have an air intake in the bottom of the cook chamber. Was this used as a grill at some point? Best to close it off when smoking.
As for the chimney, there is no real difference in the height of the opening inside the cook chamber, as long as it is above the bottom rack.
I hope this helps.