Cool game! I was a bit confused by the shop, I couldn't tell if anything was even really happening until I exited the menu and realized by "free gold" you meant that it'd be added to the game board. That's a pretty clever idea, having the items directly modify/add to the level!
Like others have said, the fake soulball is also a clever solution. Using Raycasts would definitely work, although it'd be off by a little bit because the ball moves in an arc rather than a straight line (Which tbh is probably completely fine, as it likely wouldn't be off by too much).
You could also take what you're already doing and instead of actually showing the fake soulball, just keep track of it's position every X number of seconds. Then you could pass all those points into a Line2D to draw the full trajectory. That'd be 100% accurate, although you'd still have to wait for the fake soulball to travel it's full duration. Lots of ways to solve the same problem!