I’m currently using InfluxDB + Telegraf + Grafana combination to monitor Linux systems and k3s pods. It’s basically same as Prometheus, but InfluxDB uses push model, which makes it easier to develop tools for collecting custom time series data.
For alerts and dashboards, I think Grafana is the simplest and most hassle free solution available at the moment.
Instead of single pool, I simply split my drives into tiers: cache, storage, and trash due to limited drive counts. Most R/W goes to the cheap trash and cache disks instead of relatively new and expensive NAS drives.