Ok, found the answer myself.
To get your typescript plugin to work as node_module, you need to adjust the tsconfig.json. Make sure, that everything that is important is included in the build output. Than publish or link your plugin from inside the build folder.
I used this tsconfig here as a reference: https://github.com/VirtusLab-Open-Source/strapi-plugin-navigation/blob/master/tsconfig.json