How would you design a governance system that allows people to pay for the policies they want (direct bribery) but doesn't become a plutocracy (where only the rich have a voice)?

The only problem is externality pricing.

Building https://negationgame.com

Then consider, on blockchain, all governance systems allow bribery, because all are helpless to prevent it

YES! this is exactly why I'm asking. Blockchain makes bribery impossible to prevent

This is where the thought leaders are quick to say that there's nothing that can be done, and just ignore it, forget about solving it.

Like shooting grandma, and then telling the grandchildren no worries because grandma probably wasn't going to live much longer anyway.