Thanks for bringing this up.
Issuing certs via Certificate resources currently always uses Kuberenetes Secrets to store certificates and keys.
There are a couple ideas that we're currently looking at that might be relevant to your use case, including optionally not storing private keys in Secrets and reducing cert-manager components' memory consumption.
We'd be interested to hear more about your use case- is the only reason for wanting to use another store for certificates and keys to reduce memory usage? Is this for ingress certs?