That's not how seed phrases work. The process of using a seed phrase to generate key pairs is a one way process via various hash functions. You cannot start with a private key and derive a seed phrase which produces that private key, unless you are doing something very non-standard which I absolutely would not recommend.

I would say the main benefits of a hardware wallet over a paper wallet is that they are much easier to set up and configure for the average person, and they are also much easier to spend from in the future. Paper wallets are a poor choice for the majority of crypto users, as they do not have the ability to set them up in a safe and secure fashion without doing something wrong, relying on third party software, leaking information online, etc., and will also tend to import them in to a hot wallet when they want to spend from them. Paper wallets can be very safe, but only if you really know what you are doing with them. You should be using a permanently airgapped device (i.e. not just one which is temporarily offline) to create them, and they should only ever be imported back on to this permanently airgapped device to sign airgapped transaction when you want to spend from them. I would also suggest using verified and open source wallet software such as Bitcoin Core or Electrum to generate your entropy/key pairs/seed phrase.