Cacti has some interesting features. It can be very fast, and I recall there being some third party template specific for ios-xr cpu/memory as well as a threshold plugin that has notification capabilities. Cisco's Prime Performance Monitor also seems promising, but I'm not clear on the notification part of that.
I'm no developer, so some of what my group has been using seems to always come with complications. We get stuff done, but there's frequently some previous experience that was lacking that gets in the way of forward progress. If you're starting from scratch, you might want to have a look on a broader scale. I recall recently reading this: https://workaround.org/article/tired-of-nagios-and-cacti-try-zabbix and thinking that I would've liked the room to try something new.
