I don't think injecting is worth the hassle. I've made some pretty good briskets without it. I question how much of the injection really stays in the meat during the cook. I think seasoning, temperature management, setting the bark wrapping correctly, pulling at the right temp, and resting in a cooler for a while before serving are all more important than injecting. If you're already nailing all these other things, maybe try injecting and see if it makes a difference?
Never heard of injecting with beef tallow. If you try it, please post something about the results.
Good Luck!