Navmesh Generation

I suggest you take a pragmatic approach and simplify this as much as possible to reduce need for any intense math here.

First, lets start with that you probably don't really need a Mesh or NavMesh exactly but a pool of points that all met some specific criteria. That criteria, for example, being:

• Point must be reachable from a nav mesh

• Point must represent a location of some kind of safe cover against given (player) position

  ---

So let's try that: Just points on a NavMesh. Nothing special.

5 minutes later..

Optimization

To optimize this I advise you not to raycast all of these points every frame like I did but only once to generate a companion hash map <player_spatial_hash key,int[] point indices> instead. Idea here being that you create a lookup table for every (reasonable) player position on scene start and just lookup point pools with zero additional raycasts after that.

 // NavPoints.cs
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.AI;
 using Unity.Mathematics;
 [ExecuteInEditMode]
 public class NavPoints : MonoBehaviour
 {
     [SerializeField] float3 _size = new float3{ x=20 , y=10 , z=20 };
     [SerializeField] float3 _step = new float3{ x=0.5f , y=1 , z=0.5f };
     public static half3[] points = new half3[0];
     #if UNITY_EDITOR
     void OnDrawGizmosSelected () => Gizmos.DrawWireCube( transform.position , _size );
     #endif
     void OnEnable ()
     {
         points = SamplePoints( new Bounds{ center=transform.position , size=_size } , _step , 1<<0 );
     }
     public static half3[] SamplePoints ( Bounds bounds , float3 step , int areaMask )
     {
         int3 numSteps = (int3)( (float3)bounds.size / step );
         float3 start = (float3)bounds.min + step*0.5f;
         float sampleDist = (float) math.max(math.max( step.x , step.y ) , step.z );
         var set = new HashSet<half3>();
         for( int y=0 ; y<numSteps.y ; y++ )
         for( int z=0 ; z<numSteps.z ; z++ )
         for( int x=0 ; x<numSteps.x ; x++ )
         {
             float3 vec = start + step*new int3{ x=x , y=y , z=z };
             if( NavMesh.SamplePosition( sourcePosition:vec , hit:out var hit , maxDistance:sampleDist , areaMask:areaMask ) )
                 set.Add( (half3) (float3) hit.position );
         }
         var results = new half3[ set.Count ];
         set.CopyTo( results );
         Debug.Log($"{nameof(NavPoints)}: {results.Length} points sampled ({(results.Length*6)/1000f:0.0} MB)");
         return results;
     }
 }

 // PlayerHeadLocationTester.cs
 using UnityEngine;
 using Unity.Mathematics;
 public class PlayerHeadLocationTester : MonoBehaviour
 {
     #if UNITY_EDITOR
     void OnDrawGizmos ()
     {
         RaycastHit[] hits = new RaycastHit[1];
         float3 origin = transform.position;
         for( int i=0 ; i<NavPoints.points.Length ; i++ )
         {
             float3 point = NavPoints.points[i];
             float3 dir = point - origin;
             if( Physics.RaycastNonAlloc( origin , dir , hits , math.length(dir) , 1<<0 )!=0 )
             {
                 var hit = hits[0];
                 Gizmos.color = Color.blue;
                 Gizmos.DrawSphere( point , 0.1f );
             }
             else
             {
                 Gizmos.color = Color.yellow;
                 Gizmos.DrawSphere( point , 0.1f );
             }
         }
     }
     #endif
 }