You can use a higher ohms coil and it'll be weaker.

You can even look up the ohms of various coils to choose what you like:

https://flippers.com/coil-resistance.html

But here is the deal.

The purpose of the slingshots is to STEAL YOUR GAME. If the ball just rolls down to the flippers, you'd have control of the ball, you'd have game. When the ball is propelled away from your control by the slingshots, you can't get control of the game.

I would say that no pinball comes ready to play, fresh out of the box. I'd say that the biggest thing that I want to tweak on nearly every pinball I play is the sensitivity of the slingshots.

So, I suspect that it isn't the fact that when the slingshots trigger they push the ball with too much force...

I suspect that they just trigger too often.

So, pull the plastic covers off of the slingshots.

Take a pair of needle nose pliers, and widen the gap between the contacts. Set the gap to a full 1/8 of an inch (this is too far...). I just jam the closed needlenose pliers between the blades of the leaf switch until they are bent apart. With the plastics still off, roll the ball into the slingshot. You want it to trigger when the ball hits the rubber with a bit of force, but not trigger when the ball is moving slowly, or at a grazing angle.

Bend the contacts a bit back together. The actual gap is good for most games at a little thicker than a nickel. An 1/8 inch gap is going to be noticeably dead on most games, but slingshot geometry varies. Play around with it, until you find the right balance.

As they come from the factory, a lot of games have slingshot sensitivity that is so hair-trigger that they almost reach out and smack the ball before it touches the rubber! This steals your game.

The other thing to experiment with is that if the ball is in one position on the slingshot, it'll be propelled right into the outlane. So make the switch that is triggered at that spot (the lower switch?) particularly open on the right side, while leaving the other switch a bit more sensitive. Lots of opportunities to tweak here.

I don't think I've ever felt I needed to reduce coil strength. I almost always feel that I want to adjust the slingshot sensitivity.

PinRetail is spot on here. But if you want to experiment without swapping the coil (after you make above adjustment), I guess you could throw in series a 5 ohm 10W resistor just to try approximately the same resistance.