In my experience, briskets that are very flexible when you buy them end up being very juicy at the end of the cook. I think the stiff ones are the ones that end up not being juicy like yours.
I ask my butcher for the most flexible brisket he has. Give it a try next time.
Jim - Another transplanted Texan
Former KCBS CBJ
Large and Medium Big Green Eggs , Black 18.5" WSM, Blue Weber Performer - Stainless, Green Weber OTG Kettle , Brinkmann SnP Pro, and a Stainless UDS. One retired Portable Kitchen grill.
Red Thermapen, Maverick ET-732, EdgePro Apex Sharpener.
Avatar is the original 1951 Weber Kettle