Is the Sunsynk SYNK-SG05LP1-SM2 SYNK-7.6K-SG05LP1-SM2 compatible with the Sunsynk CHNT DTSU666 (3ph + CTs)?

Yes — listed by the manufacturer.

The manufacturer's published pairing data lists these together.

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