Indefinite persistence of time-series data "as-is", is a somewhat different use-case from putting them in a data warehouse so you can efficiently do rollups to them. Timescale seems to be useful for the latter, but I'm not sure it offers too much value for the former.

I believe the state-of-the-art for plain-old Prometheus data retention is https://thanos.io/ — my understanding is that it's a Prometheus remote storage integration (https://prometheus.io/docs/prometheus/latest/storage/#remote...) that archives time-series data from Prometheus into an object-store, and then fetches/streams chunks back out from said object-store to serve requests.

You could use it locally on your NAS, by running a Minio instance on there. But IMHO there wouldn't be much point in doing that, over just keeping all the data in Prometheus's own internal storage.