I think the solution to this is both unique and trivial: you cannot trust something that is freely expandable,or did not require some amount of stake from the other party. That stake can be anything, time, work or money.
If you want to trust a review, it's needs to have required a non expandable resource from the reviewer. That amount of resource should be an optimum of what an average user would be willing to expand without missing it (so that barrier of review is low), while being prohibitively expansive if an actor want to cheat the system and generate millions of reviews.
I like your thinking, but there's a middle ground before full automation: when humans are incentivized, one way or another, to provide the biased reviews. This might be via straight-forward employment of people in lower-cost places (e.g. via Mechanical Turk) or other incentives. For example, note how a proportion of Amazon reviews are gamed and unreliable.
At the moment, the only tasks (that I can think of) that come close to the 'time-consuming-enough to not scale, but not quite annoying enough to put off committed individuals' are the various forms of CAPTCHA - which is unsurprising, given that we're discussing a form of Completely Automated Public Turing test to tell Computers and Humans Apart. (And of course, there are CAPTCHA-solving farms.)
But would people invest time in a review system that required them to complete a form of CAPTCHA regularly?
> That stake can be anything, time, work or money.
I think you'll find that money should be removed from your list. There are some untrustworthy people that have tons of money. Sadly, I think trust must be EARNED, and that requires giving effort (work) and time. You cannot buy trust with simple money.
This clearly isn't true, since if you dress and make a rich person smell homeless, they simply won't be trusted in most parts of civilization.
No, what you're talking about is POWER and AGENCY. Rich people have the power to override trust through the fact that they can operate with near impunity; so you have very little agency to not trust them. If you choose to not show trust, you may invoke their wrath.
Here's the thing. A sovereign nation can "generate" as many "unique" humans as it wants (via printing "fake" but official identities). No one would be on to them until there were more users than probable people in the country.
If you want to trust a review, it's needs to have required a non expandable resource from the reviewer. That amount of resource should be an optimum of what an average user would be willing to expand without missing it (so that barrier of review is low), while being prohibitively expansive if an actor want to cheat the system and generate millions of reviews.