Such rules have to be marketable to people in public office in order to be enacted. One way might be to have a congressional alumni committee with very high salaries and strict rules on donations (this strictness could be sold as a way to prevent alumni officials from competing against the power of active officials).

Even if it cost a few hundred million a year in do nothing salaries, it'd help the integrity of multi trillion dollar budgets.

The British House of Lords sort of used to work this way until the stigma of aristocrats being involved in commercial/social activities faded.