@stephenw10
I use Zabbix , and like it.
Note
The zabbix DB loves to be on a SSD, especially when doing "cleanup tasks".
My install (Debian VM) with around 100 monitor points - takes up 20GB diskspace including OS.
Here's linux install guide(s)
https://www.tecmint.com/install-and-configure-zabbix-monitoring-on-debian-centos-rhel/
https://www.tecmint.com/configure-zabbix-to-send-email-alerts-to-gmail/
https://www.tecmint.com/install-and-configure-zabbix-agents-on-centos-redhat-and-debian/
https://www.tecmint.com/install-zabbix-agent-and-add-windows-host-to-zabbix-monioring/
I used this one , back in time.
https://www.tecmint.com/install-zabbix-on-debian-10/