Is it safe to put my private keys in an encrypted drive AES-256 and store it on a cloud service?
I would advise against anything that you can not physically lay your hands on.
I suppose the question is: as opposed to what?
The alternative is to simply print the key and keep it on paper - in two locations in case of fire, flood or loss.
As a security feature, you could change one of the digits/letters so that, even if it was stolen by someone who knew what it was, they would not be able to use it.
If you upload the key anywhere, you’re simply creating a risk where there wasn’t one before.
I think the question is more about how secure AES-256 encryption is