--disable copy specifically disables the server side copy - doing that makes rclone fall back to download and upload so I don't think it will solve this problem.
The obvious way to solve this problem is to do a sync with --dry-run and parse the logs for the deletes.
However you could do it like something like this (untested) which lists all the files then works out which files are only in the destination and deletes them.
rclone lsf -R --files-only source: > source-listing
rclone lsf -R --files-only dest: > dest-listing
comm -13 source-listing dest-listing > files-to-delete
rclone delete --files-from files-to-delete --dry-run dest: