Yes, get it. I can not really help, but what i would do is this:
Use the simple analytic solution as proposed to make an initial guess.
Create a mini simulator that measures the error (using large timesteps and analytic segments, adding drag and magnus in each step)
Adjust the target using the found error and do some iterations over all of this, making the timesteps smaller each time so accuracy increases.
I assume drag and magnus have small effect and analytic segments / large timesteps are possible. (But i'm no sportsman )
Otherwise replicate PhysX integrator (RK4?) and use small timesteps should be still fast enough for a single ball.
Usually physics engines also allow to create a second dummy world to do such stuff more easily, if that's an option with Unity.