well I started pouring metrics into #victoriametrics and trying to get anything comprehensible from #grafana. So far it looks extremely horrible, but not entirely hopeless. 😁
But at least vmalert+alertmanager can do fancy things from the data.

as standalone program prometheus is fine (though I'm using #victoriametrics which I whole❤️edly recommend) but actually replacing the whole functionality of nagios+munin will be, erm, adventurous. Using munin is pain-free and automagic, not quite true for prometheus....
Nagios is ugly but the dependency logic works for me very well. I still ought to reconfigure alertmanager, someday.