I enjoyed Scala, but I enjoy F# much more. .NET Core is a healthy cross-plat ecosystem, many of the VSCode/F# tooling issues have been eliminated over the past few years (IntelliJ was really one of the core reasons I preferred Scala), plus Akka.NET has F# bindings. Come on in, the water's warm:

How long does it take for someone with about a decade's experience in C# to a point where they can write idiomatic F#?

5yrs ago I was roughly the same, depends how much you commit up front. I had minimal FP experience, so I bought "Domain Modeling Made Functional" [1] by Scott Wlaschin and used it for side projects / class libs first. Took about 6 months to experience the full mental shift.

Even if you eventually decide you can't commit a team to F#, it's highly worth it because doing at least one app in F# will vastly improve your C# and how you structure applications. [2] I personally just got addicted to the expressive power, it's hard to replicate with any other ecosystem given the full power of .NET under the hood. Giraffe is also a great place to start if you know ASP.NET Core. [3]

[1] https://www.amazon.com/dp/1680502549

[2] https://www.youtube.com/watch?v=US8QG9I1XW0

[3] https://github.com/giraffe-fsharp/Giraffe