The convention with acme.sh is that those exported variables will have been automatically stored in ~/.acme.sh/mydyndomain.dynu.net/mydyndomain.dynu.net.conf.
Kind of depends what your goal is. The renewal time is predictable - it's 30 days before certificate expiry.
So you could setup a recurring calendar task to check if you want to avoid the certificate expiring unexpectedly. Or using an uptime monitoring tool like Uptime Robot which can warn you about that.
You could also acme.sh --update-account to attach an email address to your Let's Encrypt account. This causes Let's Encrypt to send you multiple renewal reminders if your certificate comes close to expiring.
acme.sh also features a built-in notification option, though it is potentially the least reliable option of the three, since if acme.sh is malfunctioning, it may not work at all.