Agree with Al and Wayne. The best stoves are made of light gauge material and get up to smoke burning temp fast. 40% or more of your energy is in the smoke, burn it to gain efficiency.
Nobody yet has talked about emissions. How much smoke are you “wasting”? Guessing a lot on startup and turn-down.
Burn the smoke inside the space if you seek efficiency. Chimney tuning (a gasifier chimney) can overcome some of the problems of the “too much metal” stoves. Fire brick between the metal and the combustion area helps because you don’t need to fully heat the metal before decent operation.
For efficiency, you need to get up to temp fast, and burn the smoke.
Perfect combustion creates carbon dioxide and water. Pull all the heat out after that and all you need is a water drain ;~)
Larry Dobson did this back in 1993 for those who doubt it is possible.
Heated a 600 square foot basement with small twigs and/or hardwood pellets (mostly hardwood pellets) with a gasifier stove that saved the charcoal. Could carry the stove with one finger, ran for 8 hours ;~)
In my experience, lots of metal to heat up before the burn becomes efficient is typically THE PROBLEM when having troubles with both stoves and chimneys.
Burn the smoke, save the charcoal, living is easy ;~)