Here is a bushing (or doohickey) that I think will solve your problem. Insert from inside the rim through your enlarged valve stem hole, and it will allow a presta valve to live there with no more cutting.
https://www.jensonusa.com/Wheels-MFG...Valve-Savers-2
Also, there are some Schrader valves that have metal with threads all the way to the tube, and a little ring that screws down on it.