I thought ReasonML only had tooling for front-end use - is there a mature way to use for backend development too?

It's just an OCaml syntax layer, so yeah. You just use the normal tools and the build system just does the right thing. Interspersing reason and OCaml files in a single project works fine too.

This. Although I would not use Reason since the compiler layer, bucklescript, changed its name to Rescript to rebrand as its own frontend language and left Reason holding the bag. There is no reference to OCaml in any documentation that was once under the bucklescript project. It even created its own sytax that is different than Reason and Ocaml to something more like JavaScript.

It basically should have been a new project and have had nothing to do with bucklescript.

The worst part is that the owner of bucklescript even owned some properties that had the name "reasonml" in it (like reasonml.org and the reasonml discord group, which weren't owned by the Reason team) and then he pointed all those thing to Rescript. Just the confusion did some serious damage to Reason.

My advice is to stick to OCaml.

Yeah I don't actually like ReasonML and don't think it should exist. OCaml's syntax is great and easy to use.

Ocaml is a seriously underrated language and would be my goto choice for developing a native or webapp.

I would still use Scala for backend development, though. The JVM is kind of a drag, though.