Continuously Adding SSDs to an existing PostgreSQL Server
Have you turned on TimescaleDB's native compression, available since v1.5?  I ask because you mention ZFS/BTRFS above, which suggests you aren't employing its compression already.
Typically see 90-98% storage savings in the wild...
https://docs.timescale.com/latest/using-timescaledb/compression
Additionally, you can use TimescaleDB's attach_tablespace command to add more disks, and then new chunks are load balanced across the available tablespaces.
https://docs.timescale.com/latest/api#attach_tablespace