At what temperature should I simmer bolognese to avoid burning?
Depending on what other tasks you have for the day, consider sliding the sauce (and your Dutch Oven) into your oven for 6 hours or so. 


  Slow Cooker Setting = Oven Temp Low = 185-200F High =
  280-300F


I cook my Brunswick Stew this way at 250F (covered 3 hours, uncovered 3 hours). The last 3 hours allows the sauce/broth to thicken through evaporation. The temp allows the stew to "simmer" without repeatedly stirring, worrying over it, and zero scorching! Plus, I'm free to tend to other things. 

Related:
Technique: Slow Cooking with a Slow Cooker
Slow Cooker vs Dutch Oven: A Conversion Guide