Thanks for the clarification. I have looked into the docs and have it tried out. There is no CLI option “–renew-before-expiry” or “–renew_before_expiry”. The parameter has to be uncommented and manually set in the renewal configuration file:
sudo cat /etc/letsencrypt/renewal/freizeitkarte-osm.de.conf
# renew_before_expiry = 30 days
version = 0.26.1
archive_dir = /etc/letsencrypt/archive/freizeitkarte-osm.de
…
Hmm, peculiar … has nobody (except me) the requirement to set this value during issuing or renewal?