Fable is still very much integrated with the runtime so there’s an expectation to handle those bullet points with inline annotations.

You can build an entire application in F# and compile to JS but another option is compiling to TS and calling that F# code from your TS app. I/O and views and whatnot are written in TS and the domain model is in F#. The entire model could be nothing but pure functions and resolve to a single expression!