What are you talking about?
A clever design aspect of Let’s Encrypt is the deliberately short expiry. That forces administrators to automate the issuance and renewal process.
Not to mention that you’re not supposed to “download” the private key!
The whole idea of PKI is to generate the private key locally and then have a CA sign only the public part.
If you’re doing anything else you’ve undermined the entire purpose of the thing.
I mean you are literally -- not figuratively -- handing your secrets over to Russian hackers and complaining about the people politely showing you how to make your systems safe.
Learn about the ACME protocol and certificate automation: https://letsencrypt.org/docs/client-options/