Few appletrees are compoletely without blemishes on trunks. Those marks do not look bad. I believe you would see large fast growing branches and twigs go lack suddenly while in leaf with fireblight. As for poor fruit, apple fly
maggots in the fruit can ruin the taste or make it somewhat bitter and pithy. Clean up fallen fruit to lessen the apple fly problem. It is possible that you may not get good fruit after grafting either. I have one healthy tree that gives poor fruit and even on the grafts i put on that tree. Going to take a graft from one of the bad grafts and see it it ruins a new tree. It looks like you dont have much to lose.