This is a simple hex-edit guide - we need to change bytes in two places; afterwards, the launcher won't be able to locate "mousespeed=" and will leave the value inside vars_pc.cfg as is. (If I don't use the launcher, then the resolution gets broken)

HxD will do just fine - it's a freeware hex-editor, which you can get here: https://mh-nexus.de/en/downloads.php?product=HxD20

Start HxD and simply drag-n-drop the launcher executable into HxD.

Hit CTRL + F and search for "mousespeed=", set Search direction to All. You will find two results - we simply need to edit a single byte in any place


For each case, we will simply change a letter in the option's name by patching a byte - you can either select a byte in hex-view, or a character in the Decoded text view on the right side.

Let's change mousespeed to aousespeed - select 'm' on the right side, right-click and click "Fill selection":


Replace 70 with 61, hit OK and Save (CTRL+S):


Perform the same steps for the second case, and save modifications - the launcher will not be able to change your sensitivity setting now.