What would be a safe way to store large amounts of Bitcoin long term to ensure safety from loss?
NO. Using an online source to generate private keys is incredibly unsafe for a variety of reasons. The best way would be to produce the private keys offline either with a Hardware Wallet or private keys generated from an air-gapped computer with a Bitcoin client installed on it that is capable of functioning as a wallet. Once the private key stores bitcoins, it is then advisable to export it to a paper wallet and removed securely from the computer. Then the paper wallet is left to you as an exercise to keep safe at all times.