A cron job should be the thing you do to renew all of them automatically, and yes use "letsencrypt-auto renew" to renew all of the certificates at once. You also could use epressions and list the month, but your idea is the most compact.
This should run on the first day of every second month at 1:00.
Code:
0 1 1 */2 * /path/to/letsencrypt-auto renew
Everyone makes mistakes, but to truly screw up it takes the root password!