Rotating Two Axes for Gravity,Rotating only two axes towards Object

I am trying to simulate simple Gravity by code. I have already written a piece that determins the closest "Planet" and stores its GameObject in a Variable ( "nearestPlanet" ). I am trying to implement the Character (currently a cube) to turn towards the Planet and Add a Force towards it (Gravity :D) .

My current Solution works like this:

for making the Turn:

 void GravityTurn()
     {
 
         Vector3 targetDir = nearestPlanet.GetComponent<Transform>().position - tr.position;
 
         float step = gravityspeed * Time.delta,I am trying to simulate simple Gravity by code. I have already written a piece that determins the closest "Planet" and stores its GameObject in a Variable ( ` "nearestPlanet" ` ).

I am trying to implement the Character (currently a cube) to turn towards the Planet and Add a Force towards it (Gravity :D) .

My current Solution works like this:

for making the Turn:

 void GravityTurn()
     {
 
         Vector3 targetDir = nearestPlanet.GetComponent<Transform>().position - tr.position;
 
         float step = gravityspeed * Time.deltaTime;
 
         Vector3 newDir = Vector3.RotateTowards(tr.forward, targetDir, step, 0.0f);
 
         tr.rotation = Quaternion.LookRotation(newDir);
     }

and for Pushing him down:

 void GravityPush()
     {
         rb.AddRelativeForce(0, 0, 5f);
     }

The Gravity turn is just the Stock code from Unity to "Rotate towards".

Current Solution does not allow rotation in y axis (no mouse movement) and gets glitchy near to the top and bottom of planets. For it to work, the Object has to be rotated 90° so it doesn't face face down but bottom down

Is there a possibility to just instantly turn those two axes towards the nearest planet and leave the other axes alone. There doesnt need to be a smooth rotation

Movement of Mouse:

         tr.Rotate(0, 0, Input.GetAxis("Mouse X")*-1);

I hope I could describe the Problem (My first Question)