> Erything about the design (polling, single database) tells me that it was designed primarily to sit alongside something small.

Prometheus is designed to be "functionally sharded". You shouldn't be running one "mega prometheus". Often it's something like 1 Prometheus per-team, depending on the amount of metrics each produces.

You can use federation at lower resolutions or a one of the distributed setups (Thanos/Cortex) if you want to avoid extra storage or lower resolution that federation entails.

> But I often find that the cardinality of the data is substantially lower than even the defaults of alternatives

Not to distract, but I think you meant resolution, not cardinality. Cardinality is the metadata like labels/dimensions. Resolution is the granularity in the time.