Some monitoring systems can do more than just cert expirations. They can ensure your server is reachable and using the latest cert. Some may also check IPv4 and IPv6 if you support both.
You'd be surprised how often we see people using an old cert even though they got a new one. Or setup DNS for IPv6 yet have no way to ensure it continues working.
You might also want to review which ACME Client you use. Some of them offer options to alert of troubles renewing a cert.
There are many options to provide health checks for your system. Perhaps now is a good time to evaluate options.
Paying for good monitoring for a production site is a good plan. You'll just need to find one of the many monitoring services out there to use, rather than Let's Encrypt's service that couldn't really tell if your site was using updated certificates or not.