That's just not possible. It's why detractors never got on board with the Cloud. Until FHE is feasible, the decryption keys and plaintext have to exists in RAM eventually at some point in order even if only took be re-encrypted, if any complex work is to be done on it. Because eg, Amazon, has access to your hardware, there's simply no way to prevent them from reading your secrets out of your VM that's using their RAM.

Absolutely do what you can, but understand that it's futile to defend against your own cloud provider.