Zabbix, is pretty good monitoring software that can be used on prem. It has most of the features of solarwinds in its agent and SNMP monitoring but the default UI and integration isn’t a nice as Orion.
If you are looking into cloud based, we use NinjaOne which is RMM platform but requires a endpoint to install its NMS extension to do SNMP monitoring. It comes with a whole lot of other features and integrations such as remote access via teamviewer/Splashtop, backup, and documentation modules, etc