Uses this octree implementation: https://github.com/Nition/UnityOctree

SpawnByOctree.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class SpawnByOctree : MonoBehaviour
{
	[Header("Spawn Prefab")]
	public GameObject prefab;
	[Header("Octree Setup")]
	public float radius = 10;
	public float minNodeSize = 4;
	public float looseness = 1.25f;
	public Vector3 center;
	public bool showGizmos;
	[Header("Spawn Position")]
	public AnimationCurve positionCurve;
	[Header("Spawn Size")]
	public AnimationCurve scaleCurve;

	private Transform objTransform, rootTransform;
	private BoundsOctree<GameObject> boundsTree;
	private Bounds newObjBounds;
	private int retries;
	private float survivalChance, scale, normalScale;
	private bool enableCreateFromPrefab = true;
	private Vector3 spawnDir;
	private new Renderer renderer;
	private Fergicide.Rotater rotater;

	private void Start()
	{
		boundsTree = new BoundsOctree<GameObject>(radius * 2f, center, minNodeSize, looseness);

		rootTransform = new GameObject("CubeRoot").transform;
		rotater = rootTransform.gameObject.AddComponent<Fergicide.Rotater>();
		rotater.freq = 0.1f;
		rotater.speed = 3;

		while (enableCreateFromPrefab) CreateFromPrefab();
	}

	public void CreateFromPrefab()
	{
		if (enableCreateFromPrefab)
		{
			if (retries > 200)
			{
				enableCreateFromPrefab = false;
				boundsTree = null;
				return;
			}
		}
		else return;

		spawnDir = Vector3.zero + Random.insideUnitSphere * radius * Random.value;
		normalScale = spawnDir.magnitude / radius;
		if (Random.value > positionCurve.Evaluate(normalScale))
		{
			retries++;
			CreateFromPrefab();
			return;
		}

		GameObject obj = Instantiate(prefab);
		objTransform = obj.transform;
		objTransform.localPosition = spawnDir;
		objTransform.localRotation = Random.rotationUniform;

		renderer = obj.GetComponent<Renderer>();

		scale = (0.5f + Random.value * 1.5f);
		if (Random.value > scaleCurve.Evaluate(normalScale))
		{
			objTransform.localScale = scale * Vector3.one;
		}

		newObjBounds = renderer.bounds;

		if (!boundsTree.IsColliding(newObjBounds))
		{
			boundsTree.Add(obj, newObjBounds);
			retries = 0;
			obj.AddComponent<PerRenderedData>();
			objTransform.parent = rootTransform;
		}
		else
		{
			retries++;
			Destroy(obj);
			CreateFromPrefab();
		}
	}

	void OnDrawGizmos()
	{
		if (enableCreateFromPrefab && Application.isPlaying && showGizmos)
		{
			boundsTree.DrawAllBounds(); // Draw node boundaries
			boundsTree.DrawAllObjects(); // Draw object boundaries
			// boundsTree.DrawCollisionChecks(); // Draw the last *numCollisionsToSave* collision check boundaries
		}
	}
}