I've designed and built a high-available solution with Zabbix version 5. I introduced and deployed Zabbix in my current company, and it ended up becoming a production service within the company.
I like being able to use proxy servers for different locations. The agents are pretty cool. They're easy to roll out.
The standard out-of-the-box templates are also pretty easy to use. The integration with other learning products is also good. I have, in the past, used Slack, but we've integrated it with Microsoft Teams. We also use it for SMS with a service called Redcoat. It is very flexible.
It does what I need it to do, and my manager is very happy because it doesn't cost anything. We are nearing 4,000 hosts inside Zabbix, and we've got another 6,000 access points to add to it. We've thrown everything at it, and it has managed to keep going. I am very impressed with the tool, and I'd shake their hand very hard if I got to say the compliments to the Zabbix team. They keep improving it and doing refreshes, which is one good thing about it. There is also online information as well as books that you can purchase if you're willing to read enough. There is a lot to pick up, but it is a pretty complete solution.