If I wanted to write down my private keys by hand, and use them in Bitcoin cash wallets, how safe would it be to disconnect from the internet, open the wallet, and jot the private keys down by hand? (Let's say I'd rather send  a single private key that controls one address with a small amount of BTC rather then hand over my wallet seed to a Bitcoin cash wallet)

In other words how bad would my computer need to be infected for this to still be a potential problem?

Can these viruses still take screenshots or monitor keystrokes even if my machine is offline, and then send them back to whoever infected my computer after re-connecting to the internet?

Or am I being overly paranoid? Anyone with enough hacking knowledge who can give some detail as to what the worse viruses with the worst case scenarios could do, even if you're opening up the wallet offline?

I'm sure some people like me would want to know.