Myelin, I gotta ask...what cooking grate position did you use on the pork butt, and did you have a drip pan underneath it? If you're too close to a water pan or drip pan with liquids, the meat will get more steam than smoke, and this translates to less development of bark, and/or softer bark. I looked at the smoker images and didn't see a water pan in the stock unit, so wondered if you used a drip pan above the heating unit.
I get good bark on no foiled smokes, regardless of the smoker I use, in temp ranges from 220-240*, as long as a water/drip pan isn't too close to the meat. I use little to no sugar in my rubs for the most part, with mostly natural sugars from powdered dried apple or ground dried cherry. I don't feel that the rub is going to have a big effect on the bark for long smokes. Before my ventures with fruit in the dry rub, I still got decent bark, and didn't use sugars. I would caution you on the use of excessive amounts of added sugars for longer smokes such as butts or brisket, as they can scorch even at chamber temps as low as 225* or so.
I will say this: if you had no water/drip pan and got a soft/under-devloped bark, that typically indicates that there's not enough ventilation to get a bark (too low of flow, and too high of humidity as a result). Hate to say it but the vent hole on top looks pretty small...maybe 1 to 1-1/4" diameter...hard to judge by the pics, but it may even be smaller than that. Can't tell about the intakes on the bottom. I'm just getting this nagging feeling that ventilation with this smoker could be an issue. Being it only has a 350 watt heating element, they may have reduced the air flow (intake and exhaust) to compensate for the low output heater. I'm confused as to why they opted to go this route, but I'm not an electric smoker designer.
I think if I were in your shoes, I'd toss another butt in without mustard or any other wet additive...just dry rub, crank the temp to 250* until you hit 200* I/T and see what the bark looks like.
One last question: what is your relative (ambient) humidity like during your smokes? 70, 80%, higher? It could make a big difference with the design of that smoker.
Just trying to look at all the possible angles here...I know I may have created a bit of confusion along the way...hope that's not the case.
Eric