Get a single number for rotation (2d game) when using Quaternion.FromToRotation

Well, as long as your direction vectors only live in the x-y plane you should only get a single rotation around z. However since you do Target.position - transform.position those two positions may not be at the same z position and therefore result in a diagonal vector which would give you a skewed result. Likewise if transform.up is not a pure 2d "x-y" vector you can similar issues.

Anyways you shouldn't really use FromToRotation and then using the eulerAngle property in such a case as the resulting angle could always flip 180° since you're dealing with a quaternion. A quaternion is a "single rotation value", however all 4 components belong together. Converting this to euler angles.

It seems you just want to rotate the object so its up vector points towards the target? If that's the case you should be able to just do

 transform.up = (Vector2)(Target.position - transform.position);

The Vector2 cast makes sure there's no z component in the direction vector. An alternative would be

 transform.rotation = Quaternion.LookRotation(Vector3.forward, (Vector2)(Target.position - transform.position));