I think you left out one important part, which might lead to losing your funds! When you withdraw only a portion from your bitcoin wallet the default behaviour

I think you left out one important part, which might lead to losing your funds! When you withdraw only a portion from your bitcoin wallet the default behaviour is that that your wallet is still emptied completly. The leftover amount is transferred to a different wallet generated by the software wallet you use to generate the transaction. I am not sure if the new private key could be restored from the original private, if you would delete the software wallet after the transaction. There is no good solution. You can specify multiple recipients, so you could send the leftover back to your secure wallet. But reusing adresses after a transaction is considered insecure (I don't know why). Or you have a second secure hardware wallet, that can receive the leftover.