@gaetan JS I can't help with - but I did work through some complication with having JavaFx and Android in the same multi-module project (due to the interaction between their plugins and the fact they're both JVM based).
This may help:
https://git.chrishatton.org/chris/kotlin-multiplatform-template
Critically - use of a Gradle
attribute to disambiguate JavaFx vs Android across module dependency boundaries.