Object pool with DOTS?

Start with familiarizing yourself with GameObject to Entity conversion workflow first. You may find out that's instantiating from Entity prefabs is good enough:

 using System.Collections.Generic;
 using UnityEngine;
 using Unity.Mathematics;
 using Unity.Entities;
 using Unity.Transforms;
 
 public class HybridSpawner : MonoBehaviour
 {
 
     Dictionary<GameObject,Entity> _prefabs = new Dictionary<GameObject,Entity>( capacity:100 );
 
     World _world;
     EntityManager _entityManager;
     BlobAssetStore _blobAssetStore;
     GameObjectConversionSettings _conversionSettings;
 
     void Start ()
     {
         _world = World.DefaultGameObjectInjectionWorld;
         _entityManager = _world.EntityManager;
         _blobAssetStore = new BlobAssetStore();
         _conversionSettings = GameObjectConversionSettings.FromWorld( _world , _blobAssetStore );
     }
 
     void OnDestroy ()
     {
         _blobAssetStore.Dispose();
     }
 
     public void Spawn ( GameObject prefab , Vector3 position , Quaternion rotation )
     {
         Entity entityPrefab = ToEntityPrefab( prefab );
         Entity instance = _entityManager.Instantiate( entityPrefab );
         _entityManager.SetComponentData( instance , new Translation{ Value = position } );
         _entityManager.SetComponentData( instance , new Rotation{ Value = rotation } );
     }
 
     public void Warmup ( GameObject prefab ) => ToEntityPrefab( prefab );
 
     Entity ToEntityPrefab ( GameObject prefab )
     {
         Entity entityPrefab;
         if( !_prefabs.TryGetValue( prefab , out entityPrefab ) )
         {
             entityPrefab = GameObjectConversionUtility.ConvertGameObjectHierarchy( prefab , _conversionSettings );
             _prefabs.Add( prefab , entityPrefab );
         }
         return entityPrefab;
     }
 
 }