What would be a safe way to store large amounts of Bitcoin long term to ensure safety from loss? What would be a safe way to store large amounts of Bitcoin long term to ensure safety from loss?

Is it safe to use Bitaddress.org to generate private keys and use them to generate multi-sig address, writing private keys on a paper.
Then signing transactions from my multi-sig address on offline computer.

Is that the best way?
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.