It's really quite simple - but works pretty well. There are 3 components:

Kiosk service

A simple systemd service that starts a kiosk script.

[Unit]
Description=Kiosk
Wants=graphical.target
After=graphical.target

[Service]
Environment=DISPLAY=:0.0
Environment=XAUTHORITY=/home/pi/.Xauthority
Type=simple
ExecStart=/bin/bash /home/pi/kiosk.sh
Restart=on-abort
User=pi
Group=pi

[Install]
WantedBy=graphical.target

Kiosk script

The script in /home/pi/kiosk.sh just starts a web browser in full-screen mode pointed at my home assistant instance:

#!/bin/bash

xset s noblank
xset s off
xset -dpms

export DISPLAY=:0.0 

echo 0 > /sys/class/backlight/rpi_backlight/bl_power

LANDING_PAGE="https://homeassistant.example.com"

unclutter -idle 0.5 -root &

/usr/bin/chromium-browser --noerrdialogs --disable-infobars --kiosk $LANDING_PAGE

Display service

I have a very simple python/flask service that runs and exposes an endpoint that lets you turn on/off the display. It's called by a homeassistant automation for when the motion detector senses or hasn't sensed movement.

Here's the python - I have this started from another "kiosk.service" systemd service as well.

#!/usr/bin/env python3
import subprocess

from flask import Flask
from flask_restful import Api, Resource

def turn_off_display():
    with(open(backlight_dev, 'w')) as dev:
        dev.write("1")


def turn_on_display():
    with(open(backlight_dev, 'w')) as dev:
        dev.write("0")


class DisplayController(Resource):
    def get(self, state):
        if state == 'off':
            turn_off_display()
        elif state == 'on':
            turn_on_display()
        else:
            return {'message': f'Unknown state {state} - should be off/on'}, 500
        return {"message": "Success"}


def init():
    turn_on_display()


if __name__ == "__main__":
    init()
    app = Flask(__name__)
    api = Api(app)
    api.add_resource(DisplayController, '/display/<string:state>')
    app.run(debug=False, host='0.0.0.0', port=3000)

You can then have the HA rest action call this with "http://pidisplay:3000/display/on" or off.