DeterministicObject parent stuff (untested)

deterministicnetworkobjects.cs

public struct DeterministicObjectSpawnState {
    public byte PrefabIndex;
    public Vector3 Position;
    public Quaternion Rotation;
    // ...
}
public class DeterministicObject : MonoBehaviour {
    public void Init(DeterministicObjectSpawnState state) {
        // ... do something ...
    }
}
public class DeterministicObjectContainer : NetworkBehaviour {

    public GameObject[] Prefabs;
    public SyncList<DeterministicObjectSpawnState> Spawns = new SyncList<DeterministicObjectSpawnState>();
    private List<GameObject> _spawnedObjects = new List<GameObject>();

    private void Awake() {
        Spawns.Callback += OnSpawnsChanged;
    }

    [Server]
    public void ServerAdd(DeterministicObjectSpawnState state) {
        Spawns.Add(state);
        if (!NetworkClient.active) {
            // if not hostmode, we need to call these ourselves
            OnAdd(state);
        }
    }
    [Server]
    public void ServerRemove(int index) {
        Spawns.RemoveAt(index);
        if (!NetworkClient.active) {
            // if not hostmode, we need to call these ourselves
            OnRemoveAt(index);
        }
    }
    // etc
    private void OnSpawnsChanged(SyncList<DeterministicObjectSpawnState>.Operation op, int itemindex, DeterministicObjectSpawnState olditem, DeterministicObjectSpawnState newitem) {
        switch (op) {
            case SyncList<DeterministicObjectSpawnState>.Operation.OP_ADD:
                OnAdd(newitem);
                break;
            case SyncList<DeterministicObjectSpawnState>.Operation.OP_CLEAR:
                OnClear();
                break;
            case SyncList<DeterministicObjectSpawnState>.Operation.OP_INSERT:
                OnInsert(itemindex, newitem);
                break;
            case SyncList<DeterministicObjectSpawnState>.Operation.OP_REMOVEAT:
                OnRemoveAt(itemindex);
                break;
            case SyncList<DeterministicObjectSpawnState>.Operation.OP_SET:
                OnSet(itemindex, newitem);
                break;
            default:
                throw new ArgumentOutOfRangeException(nameof(op), op, null);
        }
    }

    private void OnSet(int itemindex, DeterministicObjectSpawnState state) {
        Destroy(_spawnedObjects[itemindex]);
        var c = Instantiate(state);
        c.Init(state);
        _spawnedObjects[itemindex] = c.gameObject;
    }

    private void OnRemoveAt(int itemindex) {
        Destroy(_spawnedObjects[itemindex]);
        _spawnedObjects.RemoveAt(itemindex);
    }

    private void OnInsert(int itemindex, DeterministicObjectSpawnState state) {
        var c = Instantiate(state);
        c.Init(state);
        _spawnedObjects.Insert(itemindex, c.gameObject);
    }

    private DeterministicObject Instantiate(DeterministicObjectSpawnState state) {
        var prefab = Prefabs[state.PrefabIndex];
        var go = Instantiate(prefab, state.Position, state.Rotation, transform);
        return go.GetComponent<DeterministicObject>();
    }

    private void OnAdd(DeterministicObjectSpawnState state) {
        var c = Instantiate(state);
        c.Init(state);
        _spawnedObjects.Add(c.gameObject);
    }
    
    private void OnClear() {
        foreach (var spawnedObject in _spawnedObjects) {
            Destroy(spawnedObject);
        }
    }
}