Consider adding ANOTHER valve
Many years ago when my plumber was doing some work (I think water heater replacement, with no separate shutoff valve for the water heater), he found that the existing old main shutoff valve would only cut ~ 90%. So he turned off the water at the meter (which also only worked around 90%) and then, with a relatively slow flow of water coming through, cut the pipe and installed a second valve, something like this 1/4 turn ball valve.
In my case, the old valve and new valve are both inside the house. Since your existing valve is outside you might want to install a new valve inside if there is an accessible location before the pipe splits off to different parts of the house.