Maybe you can also check #opensource

I already did, but the subject is too broad, and there's not much discussion there. I expect #library-development to be more specific than #opensource. The goal is to help library developers and critic library users like me to share their knowledge and experience doing Kotlin libraries, and reach for help when working on a library

Broad topic is fine while there is not too much message traffic. Separating topics too early doesn’t help them kick off and my observation is that such early-separated channels often suffer from not enough people interested. Even #opensource itself is not very popular despite quite a number of open source being done in Kotlin.