I would highly recommend looking at Cortex (or Grafana Labs' fork, Mimir) instead of Thanos.

It doesn't rely on reading the Prometheus WAL (rather, it uses Prometheus's built-in remote-write functionality). This is both less fragile and has the benefit of being about to run Prometheus in stateless mode (or replace it entirely with something like Grafana Agent, vmagent, etc.

Grafana Labs has done a proof of concept showing Mimir scaling to 1 billion active timeseries. There are also "real" users serving 700+ million metrics in production.