Kitchenaid Artisan 5KSM150 with serious engine issues

I’m writing from Germany after browsing and watching a lot of videos on Youtube. In Germany, I can’t find a resource for helpful troubleshooting suggestions, and I hope you can help me out: our 15 year old, true and trusted, everyday - in - use Kitchenaid Artisan 5KSM150 is giving me more and more troubles: the engine starts sometimes, sometimes not. If I screw out the carbons (still about 16 mm) and reinstall them, it will work for 3 - 4 starting attempts. If it runs, the engine won’t hold its speed (changing and / or decreasing speed) and starts to smell weird (without getting hot). From the middle and front end there are strange grinding noises. I replaced the worm gear 2020 (used original parts) and regreased the whole thing. I used your video to dial in the basic and 6th gear speed, and could see the governor working. What do you suggest / which parts should I replace?

Thanks a lot, very best regards from the Baltic sea and stay safe

Ralph