Coalitions are pretty static in most parliamentary democracies except sometimes when forming governments post-election.

The 51% is for the coalition, not the party. That’s what you’re missing. CoC Republicans for example have temporarily sacrificed their immigration policies to retain legislative influence - and they are a check on the Trumpist wing passing whatever anti-immigrant legislation they want, because they too cannot act without at least tacit support from the CoC wing.

The “major party” is from a systems perspective no different than a European parliamentary governing coalition.

> Coalitions are pretty static in most parliamentary democracies except sometimes when forming governments post-election.

The "except when forming governments post-election" is a major difference. It also presumes that a coalition in the legislature is required to persist for an entire election cycle rather than being formed around any given individual piece of legislation. You don't have to use a system where an individual legislator or party can prevent any other from introducing a bill and taking a vote on it.

In less partisan periods in US history, bills would often pass with the partial support of both major parties.

Moreover, the US coalitions being tied to the major parties makes them too sticky. For example, the people who want lower taxes aren't necessarily the people who want subsidies for oil companies, or increased military spending, but they've been stuck in the same "coalition" together for decades.

Suppose you want to do a carbon tax. People who don't like taxes are going to be a major opponent, so an obvious compromise would be to pass it as part of a net reduction in total taxes, e.g. reduce the federal payroll tax by more than the amount of the carbon tax. But that doesn't happen because the coalition that wants lower taxes never overlaps with the coalition that wants to do something about climate change. Meanwhile the coalition that wants lower taxes wouldn't propose a carbon tax on their own, and the coalition that wants a carbon tax to increase overall government revenue gets shot down because that would be extremely unpopular, so instead it never happens.

All countries have these problems which vary by the local political environment and history. Multiple European countries are facing particularly absurd varieties of these dilemmas because of their refusal to form coalitions with the second or third largest party in their country.

Again, it seems like the flaw is in trying to form a long-term coalition instead of just passing the bills that have enough support to pass when you put them up for a vote among all the people who were actually elected. Why should anyone have to give a crap what someone else's position is on immigration when the bill in question is on copyright reform or tax incentives for solar panels?

The coalitions do a pretty good job of representing people’s pre-existing positions. People aren’t not voting for copyright reform because their party said so, but because they agree with their party. Party discipline in the US is not nearly as strong as in most parliamentary systems.