You may not be getting enough air Into the firebox if you are only using the wheel dampers on the side. If so, start another fire and trying cracking the firebox open a little and see what happens. It also looks like the heat is coming in above the cooking grate. May need to come below what some kind of reverted plate to distribute the heat across the grate. I could be wrong but that was quick observation.
My arsenal, Stumps Clone, Two Modified offsets, open pit with grate and rotisserie,UDS, Weber One Touch Silver 18.5" kettle, Chargriller Pellet Grill, 36" Blackstone Griddle