Would you use Kotlin again for the back end? Having not yet used it for that purpose, it seems like you’d get the benefit of the JVM ecosystem along with a nice language (but perhaps too many power-features).
For that particular use-case (or related financial ones) I'd consider Rust, which was a bit too immature when I was working on this but would give you some extra speed. HFT is winner-take-all and the bar has risen significantly even in the last couple years, so if I were putting my own money at risk now I'd want the absolute fastest processing possible.
