I'm in the same boat as you.

Right now for the frontend we selected Elm.

Now for backend I've been playing with

Elixir and Haskell

Elixir has the nice syntax of ruby with all the performance of Erlang. So that makes it a good candidate however what I don't like about it is: - it isn't strongly typed... and once you get used to types and all the bugs it catches you can't go back. - although variables are inmutable they behave as if they look a lot like mutable state and I don't want to lose that warranty either.

Then there is Haskell. Haskell is great until you need to do IO. And 80% of the webdevelopment is IO... either to the database, to an API or HTML... and all the current options seem overly complicated. I tried haskell with Servant which gives strong type to an API but the syntax is ugly Used Scotty (like ruby's Sinatra) but then the URLs are strings and when definning APIs you can't specify the types. Then I tried with Yesod I don't like such heavy frameworks and try to avoid Template Hasell...

So I continue to search....

Next in my list consider is FSharp and OCaml