I wish I knew where my old truck was, I'd go and take a picture of the driver's side mirror.
I ripped it off on some thick vines many years ago. Pulled the Nutserts right out through the sheet metal.
The mirror was one of the wide swingaway type on a flat bar. It had a chrome plate that mounted on the door with the mirror bar pivoting on it.
I could not find any larger nutserts. I ground the edges of the holes down flat where they had bulged out when the serts pulled through.
Got a flat steel plate about 1/4 inch thick, and a couple of inches wide, long enough to reach past the two holes.
Drilled and tapped two holes to match the holes in the mirror mount.
Then, drilled and tapped two smaller holes between them. Then drilled two holes in the door aligned with those two.
Took the door panel off, and slid the plate up behind the two holes, and put in two screws in the smaller holes to hold the plate in position.
Then put the mirror on, and screwed the original screws into the plate.
Since I did not care about the looks, I used round head stove bolts to hold the plate in place.
They stuck up too high for the mirror mount to sit flush on the door. I drilled two clearance holes in the mirror plate for the bolt heads to come through.
That way, I could unbolt the mirror and the plate would stay in place since the bolts that held it on were not through the mirror plate.
Had I cared, I would have used countersunk heads, and maybe drilled a couple of blind recesses behind the mirror plate for clearance.
In fact, the bolt heads did not come all the way through the mirror plate. I could have made those holes blind from the back side, but it was easier to just drill all the way through.
If the nutserts are bad on yours, I would consider doing something like that. If there is enough cover to hide the back plate holding bolt heads, no one will ever be able to tell.
I could have used just one screw in the middle, and put a touch of glue to keep the plate from possibly spinning until the main bolts were in place, but I was on a roll, so I drilled and tapped two.