python thread creating in callback, not finishing

Question

I'm trying to save image buffers as .pngs with blender's python API (but this is mainly just a python question) I'm trying to speed things up by making a thread to save the image, and the function that creates the thread is being called from a callback-handler that is activated whenever the 3D screen is refreshed, here is the full code (its a little messy):

import base64, io, os, bgl, gpu, bpy, threading, time, sys
import numpy as np
from gpu_extras.presets import draw_texture_2d
from PIL import Image
import multiprocessing.pool as mpool

finalPath = bpy.context.scene.render.filepath + "hithere.png"


WIDTH = 1920
HEIGHT = 1080



offscreen = gpu.types.GPUOffScreen(WIDTH, HEIGHT)

def draw2():
    global finalPath
    global array
    global WIDTH
    global HEIGHT
    global needsSaving
    context = bpy.context
    scene = context.scene

    view_matrix = scene.camera.matrix_world.inverted()

    projection_matrix = scene.camera.calc_matrix_camera(
        context.depsgraph, x=WIDTH, y=HEIGHT)

    offscreen.draw_view3d(
        scene,
        context.view_layer,
        context.space_data,
        context.region,
        view_matrix,
        projection_matrix)

    bgl.glDisable(bgl.GL_DEPTH_TEST)
    draw_texture_2d(offscreen.color_texture, (0, -125), WIDTH, HEIGHT)

    buffer = bgl.Buffer(bgl.GL_BYTE, WIDTH * HEIGHT * 4)
    bgl.glReadBuffer(bgl.GL_BACK)
    bgl.glReadPixels(0, -125, WIDTH, HEIGHT, bgl.GL_RGBA, bgl.GL_UNSIGNED_BYTE, buffer)


    needle = threading.Thread(target=saveIt,args=[buffer, finalPath, WIDTH, HEIGHT])
    needle.daemon = True
    needle.start()
  ####  thread.start_new_thread(saveIt,(buffer, finalPath, WIDTH, HEIGHT))



def coby(scene):
    frame = scene.frame_current
    folder = scene.render.filepath
    myFormat = "png"#scene.render.image_settings.renderformat.lower()
    outputPath = os.path.join(folder, "%05d.%s" % (frame, myFormat))
    global finalPath
    finalPath = outputPath



h = bpy.types.SpaceView3D.draw_handler_add(draw2, (), 'WINDOW', 'POST_PIXEL')
bpy.app.handlers.frame_change_pre.clear()
bpy.app.handlers.frame_change_pre.append(coby)


def saveIt(buffer, path, width, height):
    array = np.asarray(buffer, dtype=np.uint8)
    myBytes = array.tobytes()
    im = Image.frombytes("RGBA",(width, height), myBytes)
    rawBytes = io.BytesIO()
    im.save(rawBytes, "PNG")
    rawBytes.seek(0)
    base64Encoded = base64.b64encode(rawBytes.read())
    txt =  "data:image/png;base64," + base64Encoded.decode()
    f = open(finalPath, "wb")
    f.write(base64.decodebytes(base64Encoded))
    f.close()

it actually does work, except when I play the timeline in blender (which calls the frame_pre callback and also the 3D view-refresh callback (not sure in which order though), most of my images are replaced, except some are not, as can be seen in this screenshot: [![enter image description here][1]][1]

I originally had all blue-planed images, then I ran the script through the thread, and it replaced almost all of them, except some of the blue-planed images still remain (seemingly at a random interval). This worked fine if I call .join() right after I make the thread, or don't use the thread at all, but seemingly the thread is the only way to make it work a lot faster.

I've been looking around for how to use threads with queues and pooling (How to use python multiprocessing Pool.map within loop, What happened to thread.start_new_thread in python 3, How can I make a background, non-blocking input loop in python?, Creating Threads in python),

SO: Why aren't all of the threads finishing? [1]: https://i.stack.imgur.com/nJCwH.png .................