Low-Effort System Monitoring?

As many of us, I manage around a dozen Linux systems: VMs that run moderately popular websites, bare metal servers for number crunshing, linux-based NASes, a media server, a radio scanner, routers. And I am struggling to find a low-effort system monitoring tool. Basically a fire-and-forget software that would report degrading disks, overheading, space and memory utilisation spikes and the like. Preferable with mobile UI.