Is the Solis S6-GR1P3K compatible with the PowerRack HV (20.48 kWh)?

Yes — listed by the manufacturer.

The manufacturer's published pairing data lists these together.

Minute Solar records Solis S6-GR1P3K with PowerRack HV (20.48 kWh) as compatible (A — Automatic). Derived from recorded specifications or compatibility groups; no manufacturer approval is recorded.

▦Manufacturer's compatibility listListed together in the manufacturer's published pairing data. A cited document reference is being added as datasheets are processed.