iPhone - prevent automatic hotspot tethering when connecting to computer
The problem isn't on your iPhone, it's on your Windows computer using the USB tether rather than Wi-Fi. You need to set the network interface priority on your Wi-Fi to be higher than the USB device, or the USB device to be lower, so that it will only use the tether if it isn't connected to Wi-Fi.

Make sure to connect the iPhone first. Directions for Windows 7 are here (these will likely work on newer versions as well):


How to change the network connection priority in Windows 7


Alternately, you could disable the connection on your Windows computer but you'll still have to manually turn it on each time so I think the above fix will provide the result you want.