Master Blaster is right, you need to lap your bore to remove the sharp edges and any rough machine marks. Like he said, steel wool wrapped on an old slightly undersized bore brush will do the job. So will one of the 3M green scrubbing pads. They can remove any sharp edges. You can also go to an auto parts store and get some fine valve lapping compound. Put it on a tight fitting patch and run it up and down your bore. You will need to add lapping compound after about every 20 strokes. Whatever you do, you must use a muzzle protector when using any abrasive material in your bore. The muzzle protector will prevent your rod from rubbing on the muzzle and changing its profile. Changing the profile will ruin the accuracy of the barrel and will require that you take it to a properly trained gunsmith to have the barrel re-crowned.
When polishing out a barrel, you will need a bit of lubricant. You can use something like a 50/50 mixture of Murphy's Oil Soap and water. Keep your patch, steel wool or whatever wet with the soap mixture to keep it working. Change your steel wool every 20 or so strokes so you will keep good sharp steel wool working for you. Do the same with the 3M pads. change them about every 20 strokes because they wear out and you will be working and not getting much done if you don't make these changes. It takes about 100 strokes to get your barrel lapped. Sometimes it takes a few more strokes. You may have to go to 200 strokes if you don't change your polishing material after every 20 strokes so you keep a good sharp material working for you. When you are doing this polishing, be absolutely sure that you are making full strokes so that your bore is getting evenly polished through its full length. When you are through polishing your bore, you must wash and flush it thoroughly to remove any abrasive material. This treatment should have your patch shredding problem solved if it is due to any sharp edges in the bore. If polishing the bore does not solve the patch shredding problem, you will need to consider changing patch material to a tighter weave or using a different lube. You can also try using felt OP wads to see if that helps.
As for the best load for your rifle, this is something that no one can tell you. Every rifle is different, just like children. However, unlike children, you cannot teach them to appreciate or like different things, you have to find out what they like. That rifle just isn't going to change for you. The absolutely best advice I can give you is to goggle "Dutch Schultz Muzzleloading Accuracy System". Send him the $20 and get his system. Then read it. Read it again, and again until you fully understand what he is telling you. Then go out to the range and do exactly what he tells you to do and in exactly the order and way he tells you to do it. Change nothing from what he tells you. If you will do this, you will end up with the absolutely most accurate load your rifle wants and with that GM barrel, his method will have you shooting more accurately than you would ever believe any muzzleloading rifle could shoot. You will then know that the few dollars that you spent on his system will be the best spent money you will ever spend on muzzleloading.
