> For something low-medium scale.

This isn't a lot to go on.

The important thing is what you're trying to instrument - hosts, applications, network, microservices, all of the above? (And then whether you want a few weeks retention, or keeping years worth.)

Grafana in front of Prometheus with node-exporter or telegraf (it can expose in prometheus mode) on the clients -- will tick a lot of boxes and is fast to get going.

Grafana in front of InfluxDB + telegraf is similar, but personally I find PromQL easier than InfluxQL.

> ... write SQL on telemetry data and plot dashboards / set alerts.

Read up about the design of TSDBs and log / tracing datastores - their design & intent heavily influences their query languages.