If someone is on vacation, they certainly don't have time to check their emails too. Beside how hard to use crontab? and a simple reminder in their phone. Download a reminder app, set the the time when the cert expires, and then use multiple crontab entries to make sure the renewing process is triggered like multiple times, if you are so paranoid about expiring, then use crontab with emailing.
30 2 * * 1 /usr/bin/letsencrypt renew | mail -s "Lets Encrypt Cron" myEmail@domain.com
So monitor app is a waste of time and effort.