since a state by state solution is basically like the honor system.

One of the strengths and also weaknesses of the US system is that states handle their own elections. If there's a problem with how elections are run, at least it doesn't become national. OTOH, you have things like this, where a state can decide to do the right thing, but another state can be dishonest and gerrymander for one party.

I wonder if there's a solution you can do where it's the honour system backed up by a nuclear option if other states don't follow the rules. Like, say that an electoral commission should create 3 maps, one gerrymandered for Ds, one gerrymandered for Rs, and one fair one. If other states play fair, the fair one will be used. If any state defects and gerrymanders to favour one party, then the map that combats that gerrymandering goes into effect.

It could be really effective if a bunch of states band together and say that they will all issue fair maps, but they all agree to switch in case another state doesn't play fair. That way you could have something like the Republicans gerrymandering Texas, or the Democrats gerrymandering California, and either way this defensive pact is activated. Maybe by doing it this way, you could even get typically Republican states to enter the pact.

The tough part then is proving that another state is gerrymandering to favour a specific party, and deciding if that crosses the threshold. But, at least it's not just the honour system.