Ranked-choice voting is not precinct summable. This means you can’t take the totals from different states and add them together. It requires a central tabulation. This means it wouldn’t work for a national election within our framework. That’s not to say ranked-choice voting is as bad as plurality. Admittedly, it’s slightly better: Maine’s Ranked Choice Voting: It’s Not Plurality.

If you want something that works and fits on the national scale, even within the electoral college, then approval voting is a simpler and more effective route. You can find a breakdown on how approval voting would work with the electoral college here: A Blueprint To Good Presidential Elections.

Totally with you on opt-out voter registration and a more compact primary. It’s unconstitutional to require a major party to allow someone who doesn’t identify with their party to vote within their party. See California Democratic Party v. Jones. That kind of primary violates a party’s First Amendment rights through their freedom of association.