Server monitoring for self-hosting is kind of hard because it basically necessitates either buying another server to monitor the first, or paying for SaaS.
From personal experience, I would just pay someone else for a SaaS monitoring solution. It will almost universally be cheaper and more reliable.
If you really wanted to run your own, Prometheus is probably the way to go. Local storage should be fine as a data store for self-hosted. Grafana can be used for dashboarding, and either Grafana or AlertManager can do the alerting component.
It’s really not all that worth it for self-hosted scale, though. Running all that in the cloud is going to cost basically the same as buying a DataDog license unless you’re at 3-ish hosts, and more than that if you’re doing clustered monitoring so you aren’t blind if your monitoring host is down.
Funny how that kind of posts is now called "DevOps", while 10 years ago it was simply called "system administration" ;-)
Besides I fail to see any DevOps tenets in it, quite the opposite: a shell script at the bottom is little in the way of reliable automation.
To me this post reads more like someone relatively new to server management wanted to share their gathered tips and tricks, i.e. me 10 years ago when I started my self-hosting journey :-D
Wish it included server monitoring as a section.