Is there a way I can get the timp gliss effect to sound the exact pitches that I want it to, or am I limited to the choices on the Timpani Glissando patch?

The recorded timpani glissandi are more for effect, so they may be a bit limited in how specific you can get with them pitch-wise.

The regular timpani patches allow for the strokes to be pitch-bent by a major 2nd via the pitch wheel controller, but that may be more easily controlled using a MIDI controller or drawn-in automation via a DAW program like Logic.