You could use Fable to transpile F# to Javascript, keeping a single language
That seems like another explosion of complexity. Source maps, mapping F# to a JS constructs when debugging, niche tooling, wrapping other JS libraries in a nicely typed F# package... IME it's best to avoid transpiling anything more exotic than typescript.
FSI is available under Linux, but I admit it's got some hairs on it. (FWIW, FSI on Windows needs a lot more TLC too).
I appear to be mistaken - I remember I was excited when the dotnet tool came out, but then they took away the REPL. FSI is mono only, right?
This is purely a matter of taste. While express.js is very accessible, I don't think it's any more so than Giraffe (a wrapper around ASP.NET Core).
Yes it's very subjective, we can agree to disagree here.
I'm all-in on F# now - once you're over the (mild) initial learning curve, you see huge dividends from the smaller codebase, static typing, fewer null-checks and drastically less testing required.
I love F#, I used it professionally and it was a great experience. And you are right, the conciseness is hard to beat. But the static typing/fewer null checks thing is easily solved in JS land with the typescript compiler.