Why would you need a publicly signed cert for a system that only accessible from a limited (known and controlled) set of systems.
You could generate an one internal cert from your PKI for an unlimited number of years (never having to renew it).
And even if you don't have an internal PKI system, any cert generated (again, for as many years as you wanted) could simply be "trusted" by those other internal systems.
Emphasis on the cert lifetime; as LE certs only last 90 days and if your process is not 100% automated, then you will have one more thing to do every couple of months (like we all don't have too much to do already).