Hi Axel, 

You already found part of the solution 👍 . When you turn off auto wind in the routeplanning options you will see the option [Reset Wind] next to it becomes enabled. What this button does is take the wind entered on the first leg, and copy that to all the other legs. The trick is now to first change the wind for the first leg to 0/0 , then open the Pro routeplanning, there choose the route tools (you used that already to turn off auto wind) and tap on the [reset wind] button. Now all legs will copy the 0/0 wind from the first leg. 

However, removing the wind does not remove the correction applied for altitude. If you want to have total control then you should adjust your aircraft Performance profile to Expert Mode (menu->Aircraft Profile->Performance Data), here you can define actual TAS values for various altitudes