Ok from what i understand most stats stop contributing to your Weapon Scaling at about 50. So what Weapons are Experienced DS players using in NG+7 when everything has much more Hp, but the Weapon stopped Scaling ?

IIRC, stats don't exactly cease contributing after 50, they just experience a rather steep dropoff in the amount they contribute to scaling after 50.

It seems like NG+7 bosses are a matter of attrition not matter what weapon your're using. That's a different story with how powerful magic is, though.

NG+whatever onward fights hardly have anything to do with your weapon anymore, it's all just a matter of your skills as a player. If you want a new weapon your best option is finding the one that overall deals the most damage at that point when fully upgraded. Really, you're just going to spend a lot of time dodging or blocking anyway. With bosses with so much health it's all a matter of having mastered their tells and having the time to waste wittling down their HP.