maybe have a cronjob keep an eye on https://www.thiswebsitewillselfdestruct.com/api/get_ttl and ping you if it gets low.

Maybe run something like this hourly, which would give about 2 hours notice:

curl https://www.thiswebsitewillselfdestruct.com/api/get_ttl -s | wc -c | grep -v 26

(Though don't run it right on the hour as too many people doing that might temporarily take down the website.)