How can I make rsync run through cron on Catalina?

Despite relying on two Time Capsules and two wired-disk Time Machine backups, I still like to backup hourly just one folder in my home directory (source code files, etc), which is particularly useful when I’m away from home.

~/ > crontab -l
45 * * * * /usr/bin/rsync -av --delete-after  /Users/john/current  /Volumes/Kingston64/

This worked fine from Tiger to High Sierra.

Now Catalina introduced a new "Full Disk Access". Adding both /usr/bin/rsync and /usr/sbin/cron is necessary to complete this cron job.

But the backups are still not happening, and no email is sent to confirm either way (whether success or failure).

How can I make rsync run through cron on Catalina?