PySide6 widgets not showing when using pyside6-uic

EDIT:
Comment points out wrongly formatted __init__, which is true. This partially fixes my issue; if I do self.ui.buttonBox.accepted.connect(self.fn) then things work fine. However in my previous solution, all widgets were addressed without self.ui. I guess my real question is whether or not there is an easy way to avoid the self.ui without manually going through every widget and moving the reference? The answer, for anyone reading this with the same complaint, is to do this:

ui = Ui_MainWindow()
ui.setupUi(self)
for field, value in ui.__dict__.items():
    setattr(self, field, value)

I was working on a project with pyqtgraph (PyQt6) but it didn't have certain methods that I require that PySide6 does have, so I am working on migrating my code over to PySide6. Previously with PyQt6 I just did

from pyqtgraph.Qt import uic
#later 
def __init__(self):
    super().__init__()
    uic.loadUi('uifile.ui', self)

And that was that; I could directly address whatever widgets I had in Qt designer with self.[Widget name].When moving to PySide6, I followed the documentation in part 1 and did this (minimal example).

from ui_test import Ui_MainWindow
from pyqtgraph.Qt import QtWidgets
import pyqtgraph as pg
import sys

class MainWindow(QtWidgets.QMainWindow):
    def __init(self):
        super().__init__()
        self.ui =  Ui_MainWindow()
        self.ui.setupUi(self)
        self.buttonBox.accepted.connect(self.fn)
    def fn(self):  
        pass
        
if __name__ == '__main__':
    app = pg.mkQApp()
    window = MainWindow()
    window.show()
    sys.exit(app.exec())

I generated the Ui_MainWindow() by doing pyside6-uic "test.ui"> ui_test.py as shown in the documentation, which generated this code:

# -*- coding: utf-8 -*-

################################################################################
## Form generated from reading UI file 'test.ui'
##
## Created by: Qt User Interface Compiler version 6.5.1
##
## WARNING! All changes made in this file will be lost when recompiling UI file!
################################################################################

from PySide6.QtCore import (QCoreApplication, QDate, QDateTime, QLocale,
    QMetaObject, QObject, QPoint, QRect,
    QSize, QTime, QUrl, Qt)
from PySide6.QtGui import (QBrush, QColor, QConicalGradient, QCursor,
    QFont, QFontDatabase, QGradient, QIcon,
    QImage, QKeySequence, QLinearGradient, QPainter,
    QPalette, QPixmap, QRadialGradient, QTransform)
from PySide6.QtWidgets import (QAbstractButton, QApplication, QDialogButtonBox, QHBoxLayout,
    QMainWindow, QMenuBar, QSizePolicy, QStatusBar,
    QWidget)

class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        if not MainWindow.objectName():
            MainWindow.setObjectName(u"MainWindow")
        MainWindow.resize(640, 480)
        self.centralwidget = QWidget(MainWindow)
        self.centralwidget.setObjectName(u"centralwidget")
        self.horizontalLayout = QHBoxLayout(self.centralwidget)
        self.horizontalLayout.setObjectName(u"horizontalLayout")
        self.buttonBox = QDialogButtonBox(self.centralwidget)
        self.buttonBox.setObjectName(u"buttonBox")
        self.buttonBox.setStandardButtons(QDialogButtonBox.Cancel|QDialogButtonBox.Ok)

        self.horizontalLayout.addWidget(self.buttonBox)

        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QMenuBar(MainWindow)
        self.menubar.setObjectName(u"menubar")
        self.menubar.setGeometry(QRect(0, 0, 640, 22))
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QStatusBar(MainWindow)
        self.statusbar.setObjectName(u"statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)

        QMetaObject.connectSlotsByName(MainWindow)
    # setupUi

    def retranslateUi(self, MainWindow):
        MainWindow.setWindowTitle(QCoreApplication.translate("MainWindow", u"MainWindow", None))
    # retranslateUi

But when I run the app, no widgets appear, and I just get an empty main window. What am I doing wrong here? I already tried replacing the pyqtgraph.Qt import with PySide6 but that didn't work.

I think your problem is caused not by PySide6 but by pyqtgraph.
I remember that I faced similar problems creating my code because pygraph examples were standalone while I wanted pyqtgraph-charts to be part of my existing Qt application.

Below is a part of code that works for me - I highlighted main differences from your approach.
I built ChartWindow in code but yes, you may create it from ui-file, not problem with it. The difference is - I used native PySide6 classes and included pyqtgraph as a widget derived from pg.GraphicsLayoutWidget (then I added axes and plot area in a constructor of this widget).

from PySide6.QtWidgets import QApplication, QMainWindow, QVBoxLayout
import pyqtgraph as pg
class ChartView(pg.GraphicsLayoutWidget):   <-- this is a widget created from PG class 
def __init__(self):
pg.setConfigOptions(foreground='#000', background='#fff')
super().__init__()   
# This class then creates axes, plot area and all other PG stuff
..........................
class ChartWindow(QMainWindow):      <--- I derive my Main window from Qt native class
def __init__(self):
QMainWindow.__init__(self)
self.layout = QVBoxLayout()
self.chart = ChartView()     <--- and include PG window in my layout
self.layout.addWidget(self.chart)
self.chart.setMinimumSize(800, 600)
...................
def main():
app = QApplication()             <--- Native Qt application object is used
chart = ChartWindow()
chart.show()
return app.exec()
if __name__ == '__main__':
main()