If you have issues with the mouse in San Andreas, such as the mouse not moving after you unpause the game, you'll love this tip. All you have to do is pause buffer for a few seconds whenever you start the game. That is, hold down ESC for about three seconds. The game will pause and unpause extremely quickly - this is known as pause buffering - and it will make the game switch to a different method of input for some reason. If you've ever noticed that your sensitivity randomly changes after playing for a little while, this is basically what we're inducing. By pause buffering, we're making that happen in just a few seconds of playing. It fixes the unresponsive mouse problem.
This was done on Windows 10 Home x64, tested on game versions v2.00 and NewSteam R2.
I hope this helped!