Not sure if my post on how I’ve open blasted for years may have any useful ideas but from my experience I’ve only gotten a finished product like that if the material was wet or damp.
Not saying thats the case because reading through your process, i don’t see how it could get wet unless it’s building condensation in the freezer but if the column is sealed up it shouldn’t 
But maybe try a run without freezing the tube.
Heres what I’m smoking on now open blasted with all diy stuff.
Here’s the post showing my method.
I’m waiting now on my 1lb closed loop extractor kit to get here, supposed to be delivered today. However I’ve blasted alot of material through my diy stuff and have gotten great results most always as long as things are dry and I don’t over heat when purging.