Generally speaking, a firebox should have 2 air inlets... One to control the air to the fire, for heat control.... A second to move the heat from the firebox to the cook chamber...
The second inlet should be directly across from the FB/CC opening.. That air inlet also adds fresh oxygen to help burn off the creosote from a smoldering pile of wood...
May I suggest you add 2 each 25mm openings at the top of door like pictured below... They should not need any adjustment... leave them open... ALSO, leave the exhaust stack damper wide open....
Something similar to what's pictured below.....
View attachment 368960..
View attachment 368961