Mallen 32, Welcome!
Lots of folks to help with your questions.
In reference to your apparent lack of char are you referring to what is called "Bark"? That is a often black crusty covering on the brisket which most people highly prize and feel indicates quality.
Remember a big piece of meat contains a LOT of water. Cooking at a lower temperature takes a while for the moisture in the middle of the meat to be released.
One of the byproducts of propane combustion is water vapor, That causes a more moist environment which generally inhibits bark formation and causes a softer exterior. Also, search out "Texas Crutch" above, that should give you a lot of insight from othrers on what they have experienced.
You could probably avoid wrapping the brisket, and thereby allowing moisture to escape. That will help in keeping the soft exterior from getting worse.
One concern on adding brown sugar AFTER cooking could be that it would burn on the grill causing bitterness and masking the flavors in your rub. Also it might actually over cook the brisket or cause it to dry out.
Also, adding a raw, turbinado, sugar or a little more brown sugar in your rub could go a long way to fixing the problem. The brown sugar will caramelize during the primary cooking process negating the need for a second cook session on the grill.
Also, just put your rub on the exterior, no mustard, oil, or liquid. Sometimes that makes the rub stick better.
Finally putting the rub on the meat earlier and letting it sit a while can cause the rub to become more of a paste, and then when you smoke for a long period of time, the moisture will evaporate, thereby leaving a bark or crust to thicken and become hard.
I know that more folks will brighten your day with great assistance.
Keep up the good work, and keep trying.
Best of luck to you!