I had this happen on my Subaru. I used med sized vise grips. It was soft metal and they grabbed nicely. The reason they rounded it off. Used to be 17mm IIRC then they went to 14 MM but with the same large bung thread.
I was going to get a grinder wheel on a small E- Drill and grind it down to the next smaller size.
I bet the vise grips may work. Hammer on it. The Impact loosens it up
2019 VW Jetta S 6MT OCI#2 7378mi-MOTUL Specific VW508.00+VW Service Filter; '17 Subaru Crosstrek CVT OC#8 60864mi-Subaru Idemitsu 0W20 + SOA Filter
You will need a special socket for rounded off fasteners. 2nd consideration is why it's rounded off. Did someone use an impact wrench? Then it's pounded on, and rounded off. Who knows what conditions the threads might be in.
Get a new magnetic drain plug and the washer. You may also need a thread repair kit by the time you get this removed.
If you raise the front end of your vehicle, and support with jack stands, you may not be high enough off the ground. Crawling under, with less than 12", does not allow you to be in a good position, or use longer tools with more leverage. Sometimes, reach and access is limited when you're trying to use power tools. When you have the vehicle up on a lift, you can stand under it. This allows you to plant your feet in a wide stance, get your body behind the tool, or maybe use power tools. Think about an impact tool. You may also need some sort of universal joint to give you the correct angle.
I would also worry about the condition of the threads after you get the drain plug out.