12lb is like a 5-bone, right?
If it’s not dry aged, I’d step the temp up in stages. Start at 103F for 3 hours or so, then 120F for another 3 hours (give or take) then go to 130F for the rest of the cook. This gives the enzymes a chance to go to go to town before the actual cooking begins. I don’t know if it really needs 24 hours, as it’s kind of a tender cut to begin with. I’d go a bit less personally. Maybe 3 + 3 + 12.
Maybe 12-15 hours total if it’s already dry aged.
The oven finish sounds fine. Make sure to pat the meat dry before applyting the rub and searing/crusting. Or a screaming hot grill, or even a blowtorch and a searzall, although the searzall on that large a piece of meat is kind of a commitment.
Use bone tip guards or you might risk a bag puncture during the cook.
Because of the size, it would probably be a good idea to pick up a cooler to cook in, the insulation will help with keeping temps even. Down at the end of this thread there are some photos of how to set it up