I had a wobble, mine was a hands off tankslapper if going through 55-60mph up or down,white lines were a buttock clencherand cracks in the road the same
-first thing headrace bearings, they are not expensive if you DIY-that made the 55-60 wobble less severe.
Radial tyres and a bigger rear wheel-Ihave used a k1100rs wheel with a 160/60/18-no wobble at all-very stable even cranked over and ok in the wet.
You have to modify a few bits to fit the wheel and fit a 1100 center stand and remove the rear caliper to fit and remove the wheel, It also lowers the rear and lowers the gearing-about 6%, not a lot but you notice it.
I have at the moment a K1100lt rear wheel(17x3 with a 140/80/17 tyre-you can go to a 150/70/17)-this needs no mods and you do not have to remove the rear caliper to fit
The other thing i have done recently was replace the swingarm bearings-a pain of a job and £30 each side and only from BMW, this helped as well.