How about getting a mouse with a “sniper button”, which lower your DPI when you hold it down. There are several brands which offer this, like Logitech, Corsair, etc.
I’m sure with some advanced mouse software you can just use a standard side button as a DPI changer, if your mouse doesn’t have a dedicated button for this.
It’s also possible to make a small script which will toggle between two sensitivity settings in game, with the press of a button. But I don’t know how effective this would be in-game.
Here’s a small script, (untested) witch should do that: https://pastebin.com/YTukCSvU
Replace X.X in SetSensitivity with the values you want and YourButton with the button/key you want to use as the toggle.
It’s probably possible to make a script which toggles the sens as long as you hold a button too. Say for example if you hold the Right Mouse Button to scope you get a lower sens, and when you release the button it goes back to normal. I’m too lazy to figure that out right now though.