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?
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.
yet rich ppl are granted more upfront trust. maybe because we assume less incentive to rob.
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.
In order to prevent hacking trust the SSO again must ensure:
- unique human, or
- resource spend
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.