I think in the current stable version they support only S3 (but they modify the chunk size to be 64MiB as far as I remember).
I hope they would implement a possibility to use a native integration too, they are growing.
However nobody stop you from use other tools for backup using the native integration like restic, Duplicacy, Duplicati, Kopia to the rclone with a native integration, etc.
I do not know, but I will pass your question to the data scientist.
For the native TrueNAS cloud sync likely the whole TrueNAS storage stored on Storj uncompressed.