Thank you so much. I went ahead and just bended the lever slightly straighter in the back part, where the washer is. And I also checked the screws that go into the feed dog bar ok the bottom. I took them out completely and adjusted the bar so both screws are screwed in evenly. Both actions together seemed to have done the trick regarding normal stitch length selection. Again a slightly moving lever on thicker fabrics but I goes I just have to live with that to a certain degree.
During cleaning the machine when unscrewing the bottom screws I thought I check the grease pot on the left hand side and found out that my service guy didn’t clean there and it has a lot of old grease. I think I might have to find another service guy in berlin. :-(