Where do I add a loot table script to?
startup or server?

I would like to use KubeJS for my minor loot table changes and not have to bring in something like OpenLoader.

Loot Tables should go into the server scripts, but keep in mind right now only block loot tables are supported (though you can edit ANY loot table .json by using the kubejs/data folder, which would basically be a virtual datapack like you have with OpenLoader)