What style of insert are you using? That is one aspect to play with.
The other one, is to rotate the bar so that it does not have excessive front clearance. The default bar clearance is designed for the minimum bore, and if you are working above that, you should be able to rotate it a bit to reduce the clearance, and improve the top rake angle all in one go.
The way the bar is clamped is also important for damping. I regularly bore with a 1/2 steel bar and 3" overhang without too much trouble. The bar is cradled in a V block holder, and setsrews on top, nothing fancy, but for sure, 6 point support. I also use relatively massive Kloppfer style toolposts on a 19" lathe