The shattering of the bullet on bone is the clue to what happened and why. Linotype bullets for instance are very, very hard and great for target shooting. Because of their hardness and strength to conform to the rifling and not strip out they can be pushed to much higher velocities than softer alloys. BUT, whenever they hit anything hard (like bone, or a rock, or a metal gong), they shatter completely, just like light match or frangible bullets do. Softer bullets can't be pushed to the same velocities, but when they hit something hard they hold together and through soft tissue hydraulic action causes them to mushroom. The alloy is more critical than the bullet profile (a wide meplat always helps though), which is why you don't want to use anything harder than wheel weight alloy for hunting bullets. I am convinced by experience that the bullet failure you experienced is 100% the fault of the alloy. A neck shot would have failed just as disastrously when it hit a vertebra. A softer bullet would have shattered the rib, sailed right on into the boiler room, done its job properly mushrooming as it made its way through, and dropped the critter in a few seconds.
I would not give up on cast, but I would focus on using bullets cast of the proper alloy with a better profile. Until you start casting your own (and it is a wonderful addiction) Bullshop will custom cast for you in the proper alloy, has a very good selection of available moulds, and very reasonable prices. Follow the link to them on the Cast Boolits Homepage. They hunt moose and caribou with cast in a wide range of calibers and can give you a lot of practical advice. If I did not cast my own, they would be my first and only choice for hunting bullets. You have a rifle that is extremely accurate with cast and that is a rare and wondrous thing. I would offer to send you some, but I have no 7mm hunting bullet moulds, only one RNGC target. (OTOH, if you get into 30cal, I have three really good ones in 150, 170, and 180gr and can send you some of them to try if you're interested.)
There is one more important reason for you not to give up on cast. If you have been successful neck shooting with jacketed bullets it says two things about you: that you are an experienced hunter, and that you are an above average marksman. Only hunters with those two key qualities can use that kind of shot effectively and prefer it. My bad experiences have come from hunters who only thought they had those qualities (one of them ignored my advice and blew the neck muscles off the side of a really nice buck with a bullet I cast for him - vertebral ricochet) and it resulted in many, many miles of tracking to finish it and then many, many more miles of dragging it out. To this day I'm grateful he wasn't shooting at a moose, and that he was honorable and ethical enough to insist on following it. We also had lots of help dragging. He learned a good lesson and now, twenty-plus years later, he fills his deer tags regularly with his own cast bullets from his rifles and revolvers. Bad experiences can teach us important lessons. Different tools require different techniques to work best.