Stop trying to make the mortar indestructible. If you specify a mix harder than the original stone, you are effectively signing the building’s death warrant. Moisture always finds the path of least resistance; if that path isn’t the mortar joint, it will be the face of the masonry itself. I have spent years in Santafé fixing 'strong' repairs from the eighties that turned seventeenth-century blocks into piles of dust. Let the lime be the sacrificial lamb.