We mix the mortar to suit your brick, not the strongest bag on the truck. The joints are supposed to be the part that gives. A mix harder than the brick around it works against the wall instead of protecting it, so I look at your brick and your old mortar before we decide what goes in.
We also take our time cutting out the old joints. Power saws are the usual tool for this, and on thin brick joints they almost always chip the brick edges and cut too far into the short vertical joints. On soft brick, a fast hand with a power tool can do real damage. Slow, careful removal keeps the edges clean, so the new joints look crisp and the brick isn't weakened.
Pointing fixes worn joints. It won't fix whatever cracked them in the first place. If I see stair-step cracks through the brick or joints that keep opening up, the house may be moving, and it may need foundation repair before fresh mortar will last. Chimneys get the same joint work, but a loose crown or cracked flue is chimney repair, and I'll tell you which one you need.