[Network Howto] synchronizing custom values with local authority

i want to synchronize custom values, like p.e. the X-position of the player via network, similar to how the networkTransform Component does it.

my code looks something like this:

[SyncVar]
float XPos = 0f;

void Update(){
 if(!isLocalPlayer){
  transform.position.x = XPos; //(simplified, of course you cannot write to transform.position.x)
  return;
 }
 //my Input handling and character controller logic
 XPos = currentXPosition;
}

now i spawn one of these for each player with local authority.

problems:

-the server-side client is synched perfectly, but somehow the connected client flickers on the server side between its actual position and XPos = 0 while moving, and defaults to Xpos = 0 when standing still. (or always is on 0)

is a syncVar the wrong way to do this? the example https://unity3d.com/de/learn/tutorials/topics/multiplayer-networking/networking-player-health?playlist=29690 seems to make all SynchVar changes on the Server. but i want to change them with local authority, just like the networkTranform

(-where / how do i set the send rate?)

-or is there an example for a custom NetworkTransform Script somewhere? so i can just grab what i need

this is what i came up with to synchronize custom variables.

potential improvements:

-only send when things change unpredictably (position / animator state change etc)

-idk any other recommendations for optimizations etc hugely appreciated

using System.Collections;
using UnityEngine;
using UnityEngine.Networking;

public class simpleNetworkTester : NetworkBehaviour {

	Animator anim;

	[SyncVar (hook = "OnServerVarUpdate")] 
	float XPos = 0f;
	[SyncVar]
	int animStateHash;
	[SyncVar]
	float animStateTime;

	void Start(){
		anim = GetComponent<Animator> ();
	}

	void Update () {
		if (!isLocalPlayer)
			return;	//only do input and logic on the own player

		Vector3 newPos = transform.position;
		newPos.x += Input.GetAxis ("Horizontal")*.1f;
		transform.position = newPos;

		AnimatorStateInfo animSI = anim.GetCurrentAnimatorStateInfo (0);

		CmdUpdateServerVars (newPos.x, animSI.fullPathHash, animSI.normalizedTime);
	}

	void OnServerVarUpdate(float newValue){
		if (isLocalPlayer)
			return;	//do not apply changes on own player
		
		Vector3 syncPos = transform.position;
		syncPos.x = newValue;
		transform.position = syncPos;    

		anim.Play (animStateHash, 0, animStateTime);
	}

	[Command(channel = 1)] //send it through the default unreliable channel
	void CmdUpdateServerVars(float newXPos, int _animStateHash, float _animStateTime){
		XPos = newXPos;
		animStateHash = _animStateHash;
		animStateTime = _animStateTime;
	}
}