Post oak and lots of time along with jalapeno salt, garlic and black pepper (and a touch of ground coffee). Always takes eternity but oh man.... this was a 15 pound packer. I like to separate the flat from the point. The flat is perfect for slicing and they finish at different times...
They definitely exist, some good info above in this thread. The link I posted her shows a tender flat cook. Down in the thread I give e some advice on picking a good packer from the store to start with. I also trim a section of the flat off to smoke at its own pace.
I typically slice the flat and shred the point but I assure you I could have pulled the flat also.
I also think that as you described it your 10 pound brisket cooked far too long. Why the oven? Not being critical but just curious. I can typically get a 13 to 16 pound packer done in about 12 hours give or take.
A brisket is like a garden, it needs lots of attention and time.
My secret is to throw the [EDIT] pointFLAT [/EDIT] out!
Others will be along soon with their schemes for getting the flat to be edible! I am pretty sure that Franklin's in Austin squirts beef tallow (basically - added fat) on their cooked briskets then wraps them in pink butcher paper and holds them at 170-degrees overnight to make the flat edible...
Strangely enough, some folks prefer the flat, but I aint one of them!
My cook time and temperatures for a standard 10.5 pound packer from Costco - 250 degrees in the smoker for 4.5 hours, wrapped in butcher paper for 3 more hours then confit style in the oven at 200 degrees for 8 to 10 hours drenched in beef tallow. Total cook time typically around 15 hours
First, I stopped using YouTube for technique a long time ago. What makes good video doesn't necessarily make good brisket ( or bread or dessert, etc).
Second, your brisket was cooked too long and there was no rest period. It was cooked the entire time.
I smoke packers to the flat, not the more forgiving point. Chamber temp isn't important. Higher temp = faster clock. Lower temp = slower clock. The physics of heat transfer doesn't matter to the meat. It knows how much heat to absorb depending on the available temp.
When the flat probes tender (whether wrapped or not) with just a hint of resistance, I'll wrap the packer in foil, put it in a pan, and toss it in my 170°F oven, which is actually 155-160°F on my middle shelf. The meat temp can be anywhere from 195-207°F depending on the grade and nature of that bovine. I'll leave it there for 3-5 hours.
Is it cooking in the 170°F oven? No. It is slowly releasing heat to the cooler oven, and the melted collagen in the meat is equalizing through the muscle. That is a true rest. I've rested on the countertop under towels, in an insulated box, and my oven. NOTHING compared to the oven. The flat melted in my mouth in a good way. Even the vacuumed-packed, frozen leftovers were tender and juicy when reheated.
I smoke Select grade packer briskets 'cause I'm cheap. Occasionally. I'll smoke Choice grade when they are as cheap as Select. First time I rested a Select grade in a 170°F oven for 3.5 hours, the result was the best brisket I ever tasted. Two more since then had the same result.