It is essentially a (weird-ass) two-round system when it comes to the presidency (and many lower offices). Party primaries exist, they overwhelmingly gave one of the nominations to a dumpster.

IMO, a runoff or single round instant-runoff wouldn't solve much. Like the sibling says, it's already sort of like that. The way they eliminate candidates still forces/incentivizes people to make choices against their actual preferences. Ideally, you could vote exactly in line with your preferences regardless of who you think has the best chance of winning and not risk making things being worse than they would have been if you "lied." I've heard many times during primaries things like "I like candidate A a lot more, but I don't think they can win the general, so I voted for candidate B". This is also an issue with approval voting, which many people seem to have a very favorable opinion of. If there is an election between <good candidate>, <bad candidate> and <total disaster shitshow candidate>, and you think there's a chance the top two might be Bad and Disaster, do you approve of bad? There's no way for you to indicate that you actually don't like Bad, but are just hoping to prevent the worst.

I think there's risk in not doing it right the first time, if you were ever to get a chance to change it.