Hi Richard; Welcome to Permies! And Welcome to the world of Rocket science!
Nice clean build you made there Rich looks good!
Is that a 6"or 8" rocket?
I think that wrapping ceramic blanket (if you have it) or other insulation (?
rock wool maybe) around the entire lower unit. Including the lower barrel will help.
Your firebricks will still need to come up to temperature. But you wont be losing heat before the riser.
What is your riser made of? Heavy firebrick? Or ceramic blanket?
If
the answer is heavy brick then you should insulate around the outside of the riser.
You want your heat maximized inside the riser creating the super hot temps that you are hoping for.
You might, if you have the head room. Add a second barrel. I added a second barrel on my shop build and it makes a huge difference in how fast it feels warmer in the room.
My brick bell takes hours to fully heat up but the barrels make the shop workable in no time.