Hi,
In the game Im working on now I have a guy throwing a sphere forward. What I remember doing is:
I added a cube game object a few metres (estimate) in front of my character and removed its collider and mesh renderer.
Then when my character spawns a sphere in his hand to throw, I make the sphere look towards the invisible cube and add force to the sphere's z axis.
The sphere spawns in front of my character and rotates to face the invisible cube, then it flies towards it and falls to the floor as it flies (as gravity is enabled).
The invisible cube almost acts like a target for it (and could be made into a crosshair by adding a canvas and image, I imagine).
I think this would work for your guitar setup, just instead of spawning the object; unparent it, turn on gravity, look towards your invisible cube and then add force on the z axis of the guitar (as it is looking forwards at your invisible cube, it will fly forward)- then adjust the height of the throw by also adding force on the y axis.
Alternatively you could just add force in the direction of the invisible cube without using the look towards action and this should also work in theory...
Also beware that colliders
may push you around a bit when thrown, I had to spawn my sphere without colliders and then enable them in mid flight. Before I did that everytime I threw a sphere it pushed my character backwards. (so in your case if the guitar usually has colliders- disable them just before unparenting and enable a second or so later. I had to play around with the timing until it stopped pushing my char backwards).
Hope this helps or something. Good luck with it!
(edit to add: I made a quick video of how my guy looks when he is throwing his spheres. My distance is set quite low on purpose:
. As always I hope this helps and is simmilar to how you want to throw things).
(edit to add: I also always wondered exactly what kinematic does. Sometimes I find it useful through experimentation, but that is it lol.)