It’s situations like this that motivated me to replace the drain plug with a valve on all of my vehicles. It’s usually one of the first things I do on a new vehicle. When I get to work later today I’ll post a link to one I recommend.
It sounds to me like your mechanic is being over cautious because he is in a position where, if his recommendation goes wrong, you might hold him responsible, so he’s better off recommending nothing than recommending your best options in this scenario. If he isn’t willing to cut new threads to fit an oversized drain valve, find another mechanic who is willing. I had a mechanic do this for me on an old motorcycle, but he was only grudgingly willing to cut new threads and install the drain valve if I agreed to take responsibility if it failed. I followed up by sealing the edges of the valve where it meets the pan with a high temperature putty epoxy. More than a decade later, it’s still holding.
EDIT: If you can find one, the item I recommend is a Fram Sure Drain kit. Unfortunately, Fram stopped making and selling them several years ago, but some are still on the market. If you can find one, buy the largest one you can find and have new threads cut to fit it: FRAM SD3 SureDrain Fast Access Oil Change Drain Plug System
The nice thing about the Fram Sure Drain kit is that it comes with a hose, making it easier to use without getting oil on your hands. Also, it’s harder to accidentally open it since you have to screw on the actuator to open the valve. The bad thing about this kit is that if you lose the actuator, you have to try to find and buy another kit.
I currently have this device installed on my motorcycle and my car’s manual transmission.
If you cannot find a Fram Sure Drain kit, or you can’t find it in the right size, I recommend this valve: Fumoto F-104 Engine Oil Drain Valve
Again, buy the largest one you can find and have new threads cut to fit it. I currently have one of these installed on my car’s oil pan, and although it is harder to open without getting oil on my hands, it works just as well. Just make sure it’s installed with the small lever pointed upward so it doesn’t stick out.