I don't know if you were able to figure this out yet or not but two things that come to mind as possible solutions. 1.) I have watched countless videos of people utilizing a smoker just like yours or similar and they always do better using wood or hard wood lump charcoal. I even found that using hardwood charcoal in my vertical cylinder style
masterbuilt smoker that it works better, burns longer, hotter, and more consistent especially if I use the water pan. Also, using wood or hardwood charcoal will cut down on the ash production significantly. 2.) I have never done this personally but a lot of people use those lava rocks or whatever they are called to help hold the heat more consistently for a longer period of time.
These are just some ideas to consider. I hope you get things worked out soon.