Hello Jan and Jörg,
What about products other than VW/Audi...? Have you considered anything else?
No, to be honest, I'm wearing the VW glasses, and I have absolutely no idea about the other diagnostic systems of different vehicle brands.
It's definitely worth considering...
@joergs: Your nice circuits with a microcontroller and display might be suitable for an online shop.
Greetings, Rainer
'If it's finished and working, I'd love to see it!'
Regarding the other vehicles, I think you'll have no choice but to deal with them. However, it's definitely challenging to stay focused and maintain an overview. Above all, having reliable expertise is extremely important!
What

might make your life easier is that, somehow, all vehicles will soon be required to have OBD-II. This gives you a unified protocol, potentially 3 adapter versions, and a unified software. You just have to see to what extent the values outputted are suitable for a detailed diagnosis and can be used for that purpose. In my OBD-II tinkering, I have already delved deeper into this topic and have come to the conclusion that it might not be enough for me! Why should a vehicle manufacturer provide detailed insight into its deepest internals via an all-around tool?
But if you like, we can discuss this in more detail via email or private message! I think it would go too far for this thread.