I think if you're putting out "Senior dev with 3+yrs experience in GraphQL and Kotlin" ads you're going to have real trouble in most markets, if not all.
Of course you shouldn't be doing that, is the thing, but that's how most job ads read.
Of course you shouldn't be doing that, is the thing, but that's how most job ads read.
Kotlin is fine, and GraphQL is fine, but when you combine those, you have a very small niche.
GraphQL and (Kotlin or C# or Java or Scala) probably isn't that small of a niche.
Edit: Also, Kotlin does have near total compatibility with Java and can use any Java library (aside from the most magical bytecode-rewriting annotation processors) so I don't understand what you're saying about having to rewrite things that exist.