Yep, that's pretty much right. I was ready for submitting my entry for compo last night.
But Gradle (or better said, the Gradle setup you get when using LibGDX's project setup tool) really doesn't pair well with newer versions of Kotlin.
I find this really, really strange, but even after an hour of tinkering, checking versions, redownloading and even setting up a complete new project and moving the code and ressources, I could still run the game in Intellij just fine, but building it as a JAR was a big "Well, no".
I was really mad, but I was so tired that I couldn't do any further that night..
However, I fixed it today! And the fix is... working at least.
I created a new project, added Maven support instead of Gradle, added all necessary dependencies by hand, moved the code and ressources, and...
Intellij refused to include a correct MANIFEST.MF in the jar!!
But that's luckily solved by manually adding it to the JAr, and now it actually works and is complete.
Phew, that's it.
I'm actually more stress-relieved that still mad about it.
Now, I'll create some sweet itch.io page for the game, and then I'm finally done!
Oh and just for the record, because of this hassle of moving to another project, I created a new repository that contains the finished version, but I will leave the old one up for clarity that it was actually done over the weekend.
https://github.com/cozmic185/LD44-Roboxery