Some of my internal stuff goes out to Let's Encrypt, so I don't worry about it at all. My internal AD stuff is set for like three years. If anyone has compromised the CA, they're already past where issuing malicious certs would be useful.
I would up your root cert expiration. You can keep the root CA offline if you're concerned about compromise.
There are also ways to run LE-style automatic renewals internally, but I've never bothered because what I've described above means I don't need it.