Or is there a way easier way that I'm not aware of?
my first obvious solution was to PHP get-category, but I see that it is impossible in Laytheme, right?

indeed I get an array with the "catid" values.

However I'm struggling to get the category names out of the id.

@franksvamp

Sorry that i cant go deeper into custom coding at the moment but in my opinion this would be best ( i'm not always right, and i'm sure you already knew this 🌝 )

I hope some other users speak up too!

For example the category data (id's) is stored in the body of your project:

You could use these to your advantage, or use jQuery to return values of categories as well

https://laytheme.com/documentation/custom-javascript.html

See what you get in the developer console when this is added:

<script>
window.laytheme.on("newpageshown", function(layoutObj, type, obj){
	console.log("layoutObj", layoutObj);
	console.log("type", type);
	console.log("obj", obj);
});
</script>

Best

Richard