A smart contract can still help. Use Shamir's secret sharing to split the decryption key. Each friend gets a key fragment, plus the address of the smart contract that combines them.

Now none of your friends have to know each other. No friend can peek on their own, they can't conspire with each other, and if one of them gets compromised, it doesn't put the others at risk. It's basically the same idea as "social recovery wallets," which some people use to protect large amounts of funds.

If you don't have any friends then as you suggest, a conceivable infrastructure would be to pay anonymous providers to deposit funds in the contract, which they would lose they don't provide their key fragment in a timely manner after the liveness signal fails. For verification, the contract would have to hold hashes of the key fragments. Each depositor would include a public key with the deposit, which the whistleblower can use to encrypt and post a key fragment. (Of course the vulnerability here is the whistleblower's own key.)

The contract should probably also hold a hash of the encrypted document, which would be posted somewhere public.

You can create a timelock smart contract requiring a future state of the blockchain to have been reached. Once that time has been reached, you can freely execute the function on the contract to retrieve the information. Tested it years ago, to lock up 1 ETH in essentially a CD for a year.

The trust is held in your own code implementation of the contract and that ETH will continue to exist and not be hard-forked or Shor'd or something.