Features:

KeepOn is free and open-source (FOSS) without any ads/tracks and no Internet use.
KeepOn allows you to keep your device's screen on for the desired duration and also allows you to return to the default settings automaticaly when the screen turns off.
KeepOn adapts to your device's configuration to easy use!
A Tasker/Locale plugin is integrated and allows you to use KeepOn functions from another compatible application!

Permissions:

- android.permission.WRITE_SETTINGS: Needed by Android to modify screen timeout settings
- android.permission.FOREGROUND_SERVICE: Needed to detect screen off events and other screen timeout modifications
- android.permission.REQUEST_IGNORE_BATTERY_OPTIMIZATIONS: Needed to prevent Android from closing KeepOn while it is running
- android.permission.POST_NOTIFICATIONS: Needed to allow KeepOn to display notification while it is running

Source code: https://github.com/twentynine78/KeepOn