Check out the lump comparison thread by @ckreef (a seasoned professional) in the product reviews section. KJ big block took almost an hour to get up to 500*F where RO took about half the time. If you’re looking to go hot, e.g., a pizza cook, you may want to try a different lump.
I came from an Akorn to a KJ Classic I (which sounds like what you have). The design of the fire grate is much different. In the Akorn, the fire grate is just a small grill grate. You know what the KJ fire grate looks like. As a noob with the KJ, it took me some time to adjust to the cast iron KJ fire grate; I had trouble getting past about 400, kinda where you are. I took a crutch approach, and threw in a grate like the Akorn’s and had no trouble reaching higher temps. “Aha,” I needed to LEARN a little, did some sleuthing here, went back to the stock fire grate, and am now conscious of keeping the air holes clear of smalls, stones, and such.
Keep at it, and Kamado onward!