How would your proposal cure the problem you mentioned of US lawyers (or others) hired by foreign governments making inordinate campaign contributions effectively on behalf of their clients?
It wouldn't stop them. But it would do two things. First, assure that their contributions were known to the public via full disclosure, not burried in PAC dollars. Second, it would dilute the influence of the wealthy by allowing many more citizens, millions I would bet, to contribute to their favored candidates up say $500 dollars at no incremental costs to them.
There is no law ever going to be written that will take money out of politics. The answer, IMHO, is to embrace the money and encourage more people to participate. If we have several million people, each dumping $500 into campaigns, the influence of the few big spenders is severly diluted.
Don't fight the "Force", Luke. ;~)