So....
From your description, it would seem to me that your chimney is too restrictive.
For a comparison, the average pellet stove has a cup of mass burning, and gets by with a three inch pipe (7.39" cross sectional area) on the average less than 8' in length,
a hi dollar version may have up to two cups burning concurrently and a four inch stack, (12.73" cross sectional area (my Breckwell has cautions against more than 12' of stack because draft may overcome the damping effect of the draft booster and allow the fire to burn out of design parameters)).
A small
wood stove with a 2'x2'x2' fire box stuffed completely full would have the rough equivalent of 20 cups of pellets (once air voids due to surface irregularities are factored in) burning concurrently and uses a 6" pipe, (28.89" cross sectional area) these stoves tightly control the draft air and will often "woof" out the door when opened because there is more fuel than air to burn it, and the open door allows an immediate flash off of the unburnt fuel.
A large stove with a 3'x2'x2' firebox stuffed full has the rough equivalent of 35 / 40 cups of pellets burning concurrently, an 8" pipe with a cross section of 50.02" will still "woof" if the door is suddenly opened and unlimited oxygen is supplied...but usually the draft is adequate to pull all the smoke up the chimney.
An open fireplace of the same dimensions as the large stove would have a clay liner (on the average ...your mason may vary!) 12"x16" with a cross section of 131".
A large Victorian era fireplace with a firebox 5' tall x 6' wide x5' deep adequate for roasting whole pigs, or suckling calfs would have a burning mass roughly the equivalent of 120 pounds of pellets burning concurrently and the chimney would have a cross sectional area approximately 3'x3' or 1296 square inches.
FWIW heat sent up the chimney is
wasted labor my Breckwell with a 4" x 3' stack extracts all the heat out of the smoke column to the point where the outside of a double wall pipe stays cool to the touch, with a 10' stack it pulls the column through the heat exchangers faster than they can extract it and grows uncomfortably hot, 140 degrees on the outside of double wall pipe. It currently heats a 1600 sq ft doublewide in North Idaho with 70's era insulation (read inadequate insulation!) comfortably (73 degrees) on the equivalent of one wheel barrow of
wood a day (40 lbs (roughly $5.00, (or 15 minutes labor daily))).
Perhaps your redesign would benefit from a better heat exchange configuration and a smaller fire where your 6" stack, (with its restrictions!) would be adequate for the volume of the smoke column.
