People can also cheat on this self-reported familiarity right? On specialized areas I don't see a solution, the colluders might as well be the only experts in the field so you have to enlist them no matter what. But from the description this doesn't seem like what's happening.
"The colluders hide conflicts of interest, then bid to review these papers, sometimes from duplicate accounts, in an attempt to be assigned to these papers as reviewers."
One critical vulnerability in the current reviewing pipeline is that the reviewer assignment algorithm places too much weight on the bids. Imagine if you bid on only your friend's paper. The assignment system, if they assign you to any paper at all, is highly likely to assign you to your friend's paper. If you register duplicate accounts or if there are enough colluders, the chance of being assigned to that paper is extremely high.
Fortunately, this is also easy to detect because your bid should reflect your expertise, and in this case it doesn't. What we showed in our paper is that you can reliably remove these abnormal bids. It's not a perfect solution, but it helps.
"The colluders hide conflicts of interest, then bid to review these papers, sometimes from duplicate accounts, in an attempt to be assigned to these papers as reviewers."