I mostly agree with your analysis, but while Devolution sounds good on paper at an abstract distance, the reality of it is going to be (and is) a lot messier.

The problem is that we don't have just Red-vs-Blue states in the US. We also have Red-vs-Blue subregions and counties within most (all?) states, and even within a nominally Red or Blue county, city, or town, you'll almost universally find a a sizable sub-population of the opposite bent.

So, to pick an example hot-button issue: Abortion Rights. If the feds decentralize this issue and let states find their own paths, you'd have ample abortion rights in the blue states and virtually no abortion rights in red states. But many of these nominally red or blue states are really, say, a 70/30 or 60/40 split by population, and so a large (but non-majority) population of each state is now living under laws they despise, which is not going to make for good long-term outcomes.

Over a long-enough time period, I suppose many would choose to move to other states and physically polarize the nation by-state, which is then going to make it easier and more natural to eventually Divorce the two sides as separate nations, and then they will almost surely have wars in the future over turf, politics, etc. But in the shorter term, most people don't have the freedom to just move states on a whim because of political change: they have families and friend networks, they have jobs to consider, financial and social costs of uprooting and moving, etc. It's still a relatively-rare privilege, among the whole US population, to be in a situation where you can easily move states as you feel like it on a political whim.

You could also imagine a path where Devolution just gets more-fine-grained - the Feds leave more to the states, the states leave more to the counties, they leave more to the cities, etc. This would allow things to differ at a much more local level. All over the country, you might find a flip of the abortion laws every 20-30 miles traveling down the highway. This reduces the costs and challenges for the individual - now they can more-easily move a short distance for a set of laws they find palatable, or simply drive across legal lines for various services/purposes. However, I think having such a large menu of divisive and important legal issues vary at such a low level would lead to a lot of chaos and friction. You'd lose a lot of the advantages of being a relatively-homogeneous country. Imagine doing business in such a country and having to pay far more attention to the more-significant legal differences in every jurisdiction. It sounds like a mess!

My current (perhaps naive!) opinion is that you can't fix this in an effective way through Devolution or Divorce - the costs are too high. I think this needs to be fixed at the per-human level: we need to become less-polarized as individuals, and elect less-polarized politicians as well.

We need to learn to disagree without being disagreeable again, and to understand where each other is coming from. We used to have that (well, to a much larger degree than we do now) decades ago. I don't think Facebook alone created this mess, but tech is certainly playing a significant role in amplifying the problem rather than solving it.

You have to reach a point where everyone can have conversations with all of their neighbors of the form "I disagree with your stance on X, but I can see why you think that way, and I want to explore where we have common ground and find a compromise solution that can make both of us reasonably happy to live alongside each other", or your society is doomed to eventually be a failed one.

Being "right" on an issue is not as important as being good neighbors to each other and respecting others' views. Perhaps your particularly strong view of a certain issue will eventually prove itself to be universally acceptable/accepted (in the sense that eventually in some future year, an overwhelming majority of the population will agree with you instead of say, just the 35% that are in your political subcamp).

However, you're not going to get there by going to war with the other 65% and demeaning and degrading them. You're going to get there by being understanding enough to have healthy relationships with them, and then perhaps minds can change more-naturally (and it may take a generation or two for some changes, but it's the only non-violent way to get there). You have to do this even if you don't think the other side is doing it, or even capable. It's the only way to have hope for a better future.