You can use the Reminder App. I think its prebuilt into Android. You can set up a reminder to make a noise every 30mins on it. To limit it, you can use Do Not Disturb option.
If you have a Samsung watch, you can set it to vibrate or make a noise periodically. I have mine set to vibrate every hour, just to remind me to shift my position/posture while at my desk. Again, Do Not Disturb limits it.
