Akeeba Panopticon, is a self-hosted, free-of-charge site monitoring software for Joomla!™ sites!
Presentation slide deck: https://www.dropbox.com/scl/fi/pw47sg00rlgk6d7m18tmz/Monitor-Your-Sites-without-breaking-the-bank.pdf?rlkey=ntoiu5oc6bz60ngf19bxi6e7x&dl=0
More information and downloads: https://getpanopticon.com