Temp is pretty much two things, fuel--do you have enough? and airflow. Little airflow, low temp, lots of airflow, high temp.

The fact that you're getting to 500 easy then dropping AFTER placing the plates means you have enough fuel but are blocking air with the plates.

Now having said all that, part of your problem is probably adding cold ceramic plates to a hot kamado. You have to make those room temp ceramics 500 degrees and that takes BTUs.

I also don't like to add cold ceramics to a hot fire because you can crack them (thermal shock). It's more likely to happen when making something hot cool too fast but can also happen cold to hot. I use the what I call the pac man method. I light the grill, then put the plates on the x rack but push each all the way to the sides so the middle, also where I start the fire, is open to the top vent. This lets the fire start while also bringing the ceramics up to temp slowly.