I love tarragon, and I bet that would work well without getting in the way. Mexican oregano or marjoram also could work well without overpowering.

Cumin is an excellent idea and can be used easily in the background. You also can consider coriander. That’s another one that goes very well with smoked beef and, used with a bit of care, will not overpower.

Also not sure on just grinding up the whole smoked brisket or trimming off the beef fat and adding some cooked pork butt for fat content.

I would not waste pork back fat on this considering that you already have all the brisket trims at your disposal. Personally I would not grind the brisket either, just cut it in small chunks (after smoking and resting). I would more than likely smoke the point with standard SPG, and just boil it down like a traditional recipe but would add carrots to the trinity (more like a modified mirepoix, so you can add some sweetness). Looking at the seasoning of the traditional boudin I would keep the thyme and add rosemary and/or bay leaves, and may add a different heat like ancho chili powder. If you like the Southwest influence maybe adding some cumin).

Clarko If what you have calls for lean pork, then I would follow suit with lean brisket.

HAHAHA! There is nothing lean in traditional boudin
We use the whole shoulder (fat and all) and always put the fat portion first when re-adding the liquid back in the the meat/rice mixture