Sounds good 
I think I can safely conclude that the problem was indeed the Light Source in the level. It has to be set to Moveable. Not static, nor stationary works with this auto-instancing. After rebulding my main project with moveable light source the fps is at its max, and the drawcalls dropped to 40. Thank you ClockworkOcean! You just saved my career! You are the best!
Ah - fixed 
It might also work if you use static lights and build after putting everything in the scene, but moveable is better
You asked from the beginning whether my lights were dynamic, but I misinterpreted it. It’s weird that moveable lights solved the problem, I thought that would be the most performance heavy option. it’s great though because I wanted a moveable sun in other projects.