Hey @GregorSondermeier, I’ve to admit I forgot about this thread until today.
We’ve actually started rolling out some improvements to our TypeScript support and are in the process of iteratively building types for Strapi APIs (including the entity service or the core factories) which we will be releasing over the next few months.
You can find the PR that contains the foundation of our new TypeScript system here
Again, I’m sorry it took this long to ping you but I hope you’ll be as excited as we’re about the future of Strapi x TS 
Have a nice day!