First let's get your questions sorted:
Which language do you think will bring me more joy?
This you will have to decide for yourself, I am a long time C# developer and have seen it grow into a very compelling platform. The language and I'd compare it more to Kotlin than Java (by a long margin). More on .NET in a bit.
say I learn F#. Is it 100% interoperable like Kotlin?
You can have 100% interop with a caveat, your F# libraries have to implement certain guidance in order to be referenced from C#. Some (dare I say most) of the differences between F# and C# are predicated on language constructs that are not available in C#. For instance F# functions that return Unit.
can they live side by side?
Can I, then, apply to .NET jr jobs after a while, for example, or is C# the holy cow?
I don't know if I take your meaning, but let me say this: Learning either C# or F# will likely force you to understand concepts such as garbage collection, primitive types, etc. which apply to all .NET languages, thus a lot of the effort you put into .NET is bound to pay off regardless of your choice.
If it is the worst and only C# is acceptable, then which language should I learn? Dart? Go?
You can't go wrong with any of these and I venture to say whether you select C#, F#, Dart or Go as your next adventure, your willingness to learn will take you to try other languages, some which mey not even exist yet!
PS2. Microsoft is working on unifying .NET. Soon there will be only one version: .NET 5! Caveat: Some features such as WinForms will still be specific to the windows environment but all of those are likely things you don't need in Mac or Linux