If you're using ClojureScript/Elm/whatever, there's going to be a lot more hassle to use existing tooling, such as e.g. test runners, syntax highlighters, etc.

There are plenty of tools for ClojureScript, including syntax highlighters and test runners. ClojureScript, like Clojure, also embraces the host platform, and can call out directly to JavaScript, so a lot of existing JS tooling is available as well. For example, the Karma test runner is frequently used.

I’m not familiar with the Elm ecosystem, but I would be surprised if they don’t have pretty good (or likely excellent) tooling as well. I’ve seen syntax highlighted Elm and it’s well known for it’s excellent error messages. Any language change is likely to involve changes in the tools you use, but I don’t think that lack of tooling is a reason to avoid either of these.

Yeah I mean, many compile-to-JS languages have pretty good ecosystems (I know Elm is one, and I believe you that ClojureScript is too), but it's always going to be a pain in the ass at some point. Hell, I've been wrestling with just setting up Jest with TypeScript not that long ago, and measuring code coverage still isn't perfect. You're not going to tell me that you never run into those sort of things with non-JS languages that compile to JS.