Is there a standard/recommended way that I can write, for example,

Observable.zip(obsA, obsB) { a, b -> a + b }

instead of

Observable.zip(
        obsA,
        obsB,
        BiFunction { a, b -> a + b }
)

? I know how to write

SomethingCustom.zip(obsA, obsB) { a, b -> a + b }

but I'm asking if there's a best practice or well-known library or something

This should solve your problem: https://github.com/ReactiveX/RxKotlin

Perfect! Just what I was looking for. Ty!