there are several reasons why your current heater configuration doesn´t work properly:
1) the recommended ratio of the lengths of feed tube, burn tunnel and heat riser are 1:2:4, So your heat riser is way on the sort side.
2) the area around the heat riser where the gases must come down again seems very narrow, normaly the minmum gap should be 2 inches / 5 cms all around. Not to forget a big enough top gap between riser and ceiling for the gases to turn direction.
3) the use of metal to contain the burning fuel has serveral disadvatages:
It conducts heat away from the fire, thus cooling the burn and limiting its efficiency.
Due to this the feed tube will also heat up and serve as a competing chimney causing smokeback.
With an unsinsualted heat riser the metal wil also conduct heat between the inner (upwards) and outer (downwards) exhaust gas path, so the heat riser gets less hot and produces less updraft and the surrounding chamber gets hotter and produces less downdraft.
If you manage to get hot and clean rocket combustion by insulating the burn area form the outside, metal will spall and disintegrate over time.
However metal is fine as a casing around an insulated burn tunnel and heat riser made of refractory materials,
I understand well that it´s tempting to construct a rocket stove with the materials you have at hand, but if you want to heat with very little wood and no smokeback and a clean exhaust you´d have to redesign your stove qite a bit.
First thing would be that you construct your feed, burn tunnel and riser of firebrick splits. There´s a mock up for a small fire brick core and heat riser that I found very nice in the first few minues of this video
This fire brick core should be wrapped either in cermic insulative fiber board or put into a bigger metal tube that is backfilled with a mixture of 1 part fireclay and 3 parts Perlite, both serving as insulation to keep the combustion super hot and clean.
Also such a " soft insulation" will protect the firebrick core from shocks while driving with your van.
And the radiating casing around all this should be big enough to not restrict the downflow of the burn gases.
I hope this is helpful.
All the best,