The bigger problem in my opinion is more about the fact that all elections that select a single winner will always end up in stupid degenerate systems like this where flaws and imperfections exist.

The best thing to do (again, my opinion) is to abolish all single winner races and have multiple winners with proportional representation. Get rid of directly elected presidents and have a prime minister selected by a proportionally representative parliament instead. All presidential systems suck, and the larger the number of people voting, the harder and harder it sucks. It’s not just a USA problem - you also see it in France and Turkey, where they also have an all-powerful president that is elected nationally and the election is a complete shit-show every time without fail. On the other hand, having a prime minister selected as the head of state from a proportionally elected parliament is a much fairer and more stable system in my opinion. It has downsides too of course, but nowhere near as bad as nationally elected presidential systems.

In any case, the situation is a potential danger, but I don’t think it’s very likely to happen. First of all, it would require all those voters in the second round to conspire a particular way, which isn’t very likely. Secondly, there’s the fact that the numbers would have to line up in a very particular way which has a very low probability of happening - tweak a few numbers here and there, and the spoiler effect vanishes. Sure, the scenario you point out is a hypothetical flaw in approval voting, but I think it’s a much smaller effect and probability of actually influencing anything - definitely nowhere near as much of a strategic voting effect as in plurality voting systems.