The built-in WP Smush compatibility skips the sync on upload for images Smush will handle and uploads them to Google Cloud Storage once Smush reports the image as optimised, so the bucket only ever holds the compressed version. It detects Smush free and Smush Pro.

Enable it on the Compatibility tab of the WP-Stateless settings page, or with the WP_STATELESS_COMPATIBILITY_WPSMUSH constant. Unlike the other image optimiser compatibilities it has no mode restriction.

Tested with Smush 3.15.1.