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There is the `sealed` class modifier which allows for exhaustive matching on possible types: https://dart.dev/language/class-modifiers#sealed
There's also a package called freezed which offers similar utility (you might already be familiar with this seeing as you mentioned pattern matching): https://pub.dev/packages/freezed
I recall there being a reason unions aren't generally implemented, though I'm not sure where I read it. It was to do with union types being unnecessarily complex for the compiler and the code they tend to produce, at no real benefit to the programmer or the resulting program. Go's lack of unions is essentially based around the same reasoning.
3. dispatch on all the arguments
the first solution is clean, but people really like dispatch.
the second makes calling functions in the function call syntax weird, because the first argument is privileged semantically but not syntactically.
the third makes calling functions in the method call syntax weird because the first argument is privileged syntactically but not semantically.
the closest things to this i can think of off the top of my head in remotely popular programming languages are: nim, lisp dialects, and julia.
nim navigates the dispatch conundrum by providing different ways to define free functions for different dispatch-ness. the tutorial gives a good overview: https://nim-lang.org/docs/tut2.html
lisps of course lack UFCS.
see here for a discussion on the lack of UFCS in julia: https://github.com/JuliaLang/julia/issues/31779
so to sum up the answer to the original question: because it's only obvious how to make it nice and tidy like you're wanting if you sacrifice function dispatch, which is ubiquitous for good reason!