What's the temperature inside the unit when you're trying to light your chip tray smoking?
I suspect the issue here is you've made a well insulated unit but it's large and has considerable "thermal mass". Once it gets to temp, your heat losses will be less than the electric power you have available, and will regulate temp fine (if you don't open the door until the very end), but it will take a LONG time to get cook temp with just the electric element. And until you get to that desired cook temperature (or at least close to at least 200F) you just don't have enough temp at your tray to start your chips.
I'd recommend putting in a remote thermometer you trust in the center of the unit. (With a fine tethered wire you feed out the door seal is fine) Turn the electric element on full. Get some charcoal going (a
chimney starter works great) then transfer to a couple pans and transfer them to a couple lower racks in your unit. Close the door and start taking temp readings. Hopefully with the added heat from the coals you can get the inside of the unit up to 250F or so in less than an hour. At that point, your chips should start smoking within 15 minutes. If not, reduce the pan spacing to .25" and try again.
As long as the chip pan is steel, it could even be touching. But make sure it's not aluminum or teflon coated, etc.
In short you have enough power, at steady-state, to get your chips smoking. But until you get the entire unit to cooking temperature, all that power is going into heating up the unit, not into heating your chips.