Fox are dead if you hit the hart or lungs with any centre fire rifle . You drop issue is how you zeroed it a 243 win with any bullet suited to it should be zeroed at 200 yards the fox at 30 yards and the one at 200 plus some will also become very dead . Unfortunately it sounds you just did not get the trajectory issues or wind effects or perhaps ( and i cannot guess on this one ) the accuracy was the issue . You do also need to check on paper when changing ammo as some rounds will have a different zero - high or low or perhaps left or right of targetI’ve had my arm twisted to shoot some foxes round the sheep by the farmer.
I’ve been using the 243 100gr Norma white tails I bought for deer (the excellent rws 100gr were out of stock), which are a bit rubbish generally (3.5” plus drop at 200m and a little anemic) and I won’t be restocking with those for deer or foxes, but for now I’m zeroed in and using them.
I’m sure the sensible choice is varmint type ammo, even if I get sorted for next year now. What's go to for you foxing pros? Flat shooting and destructive is what I’m after.