'async' is not a keyword in F#, it's a builder instance no different to the ones that you can create. It's just built in to the standard library.
The return statement is only required if you want to return something form the computation expression. In your example you use async { let! x = f(); return x}, which can be reduced to async { return! f()}, which can be reduced to f().
The rest is your opinion that I don't agree with.
The distinction in this case is utterly meaningless. This is about the ergonomics of the language. Which are lacking the minute to break out of pure functional land
Read this first: https://learn.microsoft.com/en-us/dotnet/fsharp/language-ref...
There are multiple alternate asynchronous computation expression implementations which give different ergonomics and behavior (like https://github.com/TheAngryByrd/IcedTasks). There's an entire CE extension to specifically enable this kind of convenience and flexibility too: https://github.com/fsharp/fslang-design/blob/main/FSharp-6.0...
None of this is possible in C#, at least without jumping through many hoops and ending up with extra boilerplate (and this is okay, C# has enough of its own complexity). As cjbgkagh noted, providing this type of control is the whole point and what makes F# so powerful.