It can feel grainy, but I only really notice that if I'm just twiddling the wheel without driving. When I'm actually driving I don't really notice this.
As for the FFB settings this is how I understand them:
FFB Strength - this is effectively the self aligning torque - basically this is how much force the wheel will deliver as the front wheels try to return to going in the direction of the cars motion.
FFB Sensitivity - this is how resistant the wheel will be to initial change. So for example, a setting of 10 will give you a stiffer wheel when you want to make a small adjustment while going along a straight. A setting of 1 will give you a looser wheel when making the same adjustment.
FFS sensitivity does not appear to be in anyway related to surface detail, in fact to feel more detail I've found that you need to lower the FFB strength if anything.
As for catching slides, well that's just practice. it really is a balancing act between throttle, brake, and steering, and you do need to reduce your countersteer before the slide stops, otherwise you will get slapped in opposite direction as the rears regain grip. When I'm sliding I often find myself rapidly increasing and reducing countersteer 2 or 3 times and modulating the throttle/brake while "feeling" the car out of the slide. Once you get it you'll be smiling from ear to ear
. I've found this as a general rule of thumb - the stickier the tire, the more difficult controlling a slide is, so bear that in mind, and also it will vary from car to car.
Don't give up on the G29, it may not be the best wheel out there, but it's 10 times more immersive than a controller.