I just use the built-in email function that comes with mdadm. If a drive fails, I’ll know right away and replace it with a spare. You do need your server to be able to send emails with something like postfix.

If you have hardware RAID, there’s often a monitoring tool that comes with it or at the very least a command-line utility that can report the RAID state which you can then use in a script.

I managed to fit an entire Matrix Synapse server on one of those. It works surprisingly well! You will need a domain for it though.