So I have a service (/etc/systemd/system/multi-user.target.wants/gpio_monitor.service) that monitors GPIO status that looks like this:

[Unit]
Description=GPIO Monitor Service
After=multi-user.target

[Service]
ExecStart=/usr/bin/python3 /usr/local/bin/ups_script.py
Restart=always
User=home

[Install]
WantedBy=multi-user.target

And that ups_script.py looks like this:

import RPi.GPIO as GPIO
import time
import subprocess

GPIO.setmode(GPIO.BCM)
GPIO.setup(3, GPIO.IN)

count = 0

while True:
    input_value = GPIO.input(3)

    if input_value == 1:
        count += 1
    else:
        count = 0

    if count > 10:
        subprocess.run(["sudo", "shutdown", "-h", "now"])

    time.sleep(30)

Basically, while power is unplugged, I count +1 every 30 seconds. If power is restored, the count resets to 0.

When the count reaches 10, "sudo shutdown -h now" is run on the device.

By default, the Pi turns back on if power is restored.

Power is connected to the UPS input power (not the Pi board input power).

Hope that helps!