Interesting question. I would also like to find a good solution.
At the moment I am using statuscake.com to monitor the external availability of my mango server, though it will really only verify that the web-server is running.
I also use LibreNMS to chart my server stats - memory, disk, CPU, and have been meaning to set up alerts for these.
Perhaps something like a randomly changing watchdog value - which is posted to a remote server using the HTTP Publisher module, then a PHP or python listener script on the remote server alerts you if the value doesn't change in a while.
Or, set up a separate Mango server (on a VPS or something, which should be only a few dollars a month) and use the Persistent TCP publisher and listener modules to send a random number etc. across. You could make the generation of the random number quite complicated (using virtual data sources, meta points, point links, etc.) so that if any sub-system of mango breaks it will stop the value changing. On your remote mango server you could just set up a 'no change' alarm to email you.