顺序执行 2 个函数（仅在第一个函数完成后才执行第二个函数）

在我看来，你不需要额外的线程。
一切都很快完成，您可以测量方法执行的
时间。мсfunc_click_photo()

该类QTimer提供重复定时器和一次性定时器。

void QTimer:: singleShot (int msec, const QObject *receiver, const char *member)

该静态函数在给定时间间隔后调用该槽。

QTimer.singleShot(300, self.func_updater)

func_updater 该示例程序将通过自动启动插槽300 миллисекунд。

主要.py

import fnmatch
import os
import sys
import time
#import threading
#from win32comext.shell import shell, shellcon

from PyQt5.QtCore import Qt, QTimer
from PyQt5.QtGui import QPixmap
from PyQt5.QtMultimedia import QCamera, QCameraViewfinderSettings, \
    QCameraImageCapture, QCameraInfo
from PyQt5.QtMultimediaWidgets import QCameraViewfinder
from PyQt5.QtWidgets import QWidget, QApplication, QLabel, QVBoxLayout, \
    QGridLayout, QGroupBox, QScrollArea, QPushButton, QSizePolicy


class Main(QWidget):
    def __init__(self):
        super().__init__()
        self.setMinimumSize(300, 300)
        self.setWindowTitle('Test')

        self.available_cameras = QCameraInfo.availableCameras()
        self.viewfinder = QCameraViewfinder()
        self.viewfinder.setSizePolicy(
            QSizePolicy.Expanding, QSizePolicy.Expanding)

        grid = QGridLayout()
        grid.setObjectName('main_gridlayout')
        grid.addWidget(self.update_photo_btn(), 0, 0, Qt.AlignCenter)
        grid.addWidget(self.viewfinder, 1, 0, Qt.AlignCenter)
        grid.addWidget(self.showImg(), 2, 0, Qt.AlignCenter)
        self.setLayout(grid)

        self.select_camera()

    def update_photo_btn(self):
        but = QPushButton('Сделать фото и Обновить')
        but.setFixedSize(200, 50)
        but.clicked.connect(self.thread)
        return but
    
    def thread(self):
#        task_1 = threading.Thread(target=self.func_click_photo)
#        task_1.start()
#        task_1.join()
#        print(f'end task_1: {task_1}')

        self.func_click_photo()                                      # +++ <----

#        task_2 = threading.Thread(target=self.func_updater())
#        task_2.start()
#        task_2.join()
        
        QTimer.singleShot(300, self.func_updater)                    # +++ <----

    def func_click_photo(self):
        millitime = round(time.time() * 1000)
        timestamp = time.strftime(f"%d%m%Y-{millitime}")
        
#        self.capture.capture(os.path.join(f"test-{timestamp}.jpg"))
        self.capture.capture(os.path.join(
            'D:/_Qt/__Qt/Test/testtest/test/',                 # установите свой путь
            f"test-{timestamp}.jpg"
        ))

    def func_updater(self):
        grid = self.findChild(QGridLayout, 'main_gridlayout')
        box = self.findChild(QGroupBox, 'main_groupbox')

        if grid is None:
            print('Некуда вставить новый QGroupBox!')
        elif box is None:
            grid.addWidget(self.showImg(), 2, 0, Qt.AlignCenter)
        else:
            grid.removeWidget(box)
            box.deleteLater()
            grid.addWidget(self.showImg(), 2, 0, Qt.AlignCenter)

    def select_camera(self):
        self.camera = QCamera(self.available_cameras[0])
        self.camera.setViewfinder(self.viewfinder)
        self.camera.setCaptureMode(QCamera.CaptureStillImage)
        self.camera.start()

        supported_formats = QCamera.supportedViewfinderResolutions(self.camera)
        height_cam = int(supported_formats[0].width())
        width_cam = int(supported_formats[0].height())

        self.setting = QCameraViewfinderSettings()
        self.setting.setResolution(height_cam, width_cam)

        self.camera.setViewfinderSettings(self.setting)
        self.capture = QCameraImageCapture(self.camera)

    def find(self, pattern, path):
        result = []
        for root, dirs, files in os.walk(path):
            for name in files:
                if fnmatch.fnmatch(name, pattern):
                    result.append(os.path.join(root, name))
        return result

    def showImg(self):
    
#        path = shell.SHGetKnownFolderPath(shellcon.FOLDERID_Pictures)
# +++           v^v^v^v^v^v^v^v^v^v^v^v^v^v^v
        path = 'D:/_Qt/__Qt/Test/testtest/test'                # установите свой путь
        
        all_image = self.find('*.jpg', f'{path}')
        pix_dict = []
        for image in all_image:
            pxm = QPixmap(image)
            pix_dict.append(pxm)

        box = QGroupBox()
        box.setObjectName('main_groupbox')
        box.setStyleSheet("border: 0;")

        scr = QScrollArea(self)
        pnl = QWidget(self)

        vbox = QVBoxLayout()
        for image in list(reversed(pix_dict)):
            photo_label = QLabel()
            photo_label.setAlignment(Qt.AlignCenter)
            photo_label.setMaximumSize(200, 120)
            photo_label.setPixmap(image.scaled(
                photo_label.width(), photo_label.height(), Qt.KeepAspectRatio))

            vbox.addWidget(photo_label)

        pnl.setLayout(vbox)
        scr.setWidget(pnl)
        scr.setWidgetResizable(True)

        group_box_layout = QVBoxLayout()
        group_box_layout.addWidget(scr)
        box.setLayout(group_box_layout)
        return box


if __name__ == "__main__":
    App = QApplication(sys.argv)
    w = Main()
    w.show()
    sys.exit(App.exec_())