A congressional trust that any representative can pay into, that is only allowed to invest in wide US-based index funds (or it could build its own index fund) or US bonds, etc. Basically: Allow them to invest in the future of the country they're managing. Don't limit it to only politicians, either. Let everyone invest if they want.
Honestly, at this point we just need to dissolve the US in its entirety. I no longer believe it can be saved. We need to grant all 50 states full independence. They can then come back together and form whatever new nation or nations they want. The existing union is broken beyond all repair. It can no longer be salvaged.
What actually happens if the Union splits is that corporate interests start buying states. If you think we live in a cyberpunk dystopia now, just imagine what would happen if Amazon owned Mississippi through Georgia.
Well, they can be responsible for their own sins. It's called personal responsibility. Give all 50 states independence. If some states choose to become the next Belarus, that's on them. That's what independence means - responsibility for your own salvation or damnation.
None of the 50 states currently have a standing military. What if those corporate interests don't take no for an answer, and decide to hire private military contractors to conquer it by force? With the collapse of the U.S. military, there'll be plenty of mercenaries available. What if, after consolidating their power in their new former U.S. holdings, they decide that they want more and push for expansion? At that point, the corporations can have their own professional armies, made up of former U.S. citizens, fight against the remaining "independent" states.
I'm not saying your initial premise is wrong, but you haven't fully looked at the consequences of releasing ~43 incredibly weak nations into the world with no treaties or alliances protecting them. It guarantees conflict. You'd have maybe seven states masquerading as the "real United States," and complete chaos.
Realistically we wouldn't just do this overnight. You would take a couple of years to work all the details out. Peaceful dissolution of nations has happened many times before. You don't just shut down the old empire like turning off a light switch. But there would be existing institutions to build off of. The existing state national guards could be expanded to serve as full military units.
As far as doing this Constitutionally, the process is a bit dubious. But really, it doesn't matter. If you're at the point of the population being willing to voluntarily dissolve the country, you simply ignore the old constitution entirely. If a president and Congress were elected with a mandate of dissolving the US entirely, they could simply do it and there wouldn't be anyone to stop them. Hell, you could probably do this just by electing a president on a platform of dissolving the country. Yes, it really wouldn't be constitutionally valid, but in these kind of situations, that's not really relevant anymore.
That would mean helping the American population, and that isn't good for corporate interests, so "Sorry Jack, I did all I could, I pardoned my son for his crack addiction!"