@darius if you want to stick with nginx, try https://nginx.org/en/docs/http/ngx_http_acme_module.html which will handle LE certificates directly within nginx. Caddy or Apache do the same if you can/want to change your web server.
As do monitoring, updown.io does check certs and emails you if they are about to expire

@darius is "have you looked at Caddy?" an option?

@darius I might be missing something here but, at step 1, run the script that cron will run in order to do your initial certificate setup. If that works then it should work a month later when cron runs it. The only variable is if you've actually set cron up to run the correct script at the correct time, which is admittedly tricky.

To solve that problem, if you're on debian or a derivative, put your script in /etc/cron.monthly/ and chmod +x it. Debian has a built-in cron rule to run everything that's executable by root in that directory once a month. Then test it by running sudo /etc/cron.monthly/your_script. If it runs ok and sets up the certificate then it should work when cron runs it.

Also, answering the question you didn't ask, iirc letsencrypt actually recommend you run certbot every day. At least on my servers it runs every day and I will have done that for good reason. There is an /etc/cron.daily for that purpose on debian.

@darius do your first certificate deployment with your cronjob script. Add to cron only after it works. (Basically intentional „test in prod“). If you run certificate renewal from a major script which runs bespoke ones: monitor the first one for failure (eg by setting a working email alias for „root“ in your /etc/aliases and setting „set -e“ or other hard fail options in your major script).

@darius
1. Put everything behind Caddy
There is no step 2. I know it doesn't work for every use case, but it's super hand for me

@darius apologies if this is unhelpful but honestly the main reason I switched from nginx to caddy is that caddy jumps straight to step 4 for you

I did not know about out-of-band monitoring services for this! Makes sense that they exist... might have to try one

@darius I typically pair auto-renewal with an out-of-band monitor via something like Uptime Kuma or Zabbix - either of these can check cert expiry and alert if the expiry date is within some range.

This doesn’t fix script errors, but will help to catch all manner of failure modes.

@darius never make mistakes

in all seriousness though, does letsencrypt have a sandbox environment? or are you actually verifying failures at 30/60/90 days before the observable failure on day 91?