I'm running xtr/1st/xo1/e13 11 speed mix on 2 of my bikes. Ive been happy with this mix for several years.
That said, I recently swapped a 12 speed SLX drivetrain off an ebike (because I know the torque would eat that stuff for breakfast) on to my long travel rig. I find it ridiculously impressive for the slx level. So long as my 11 speed xtr shifters keep working I'm not about to trade them in but I was very impressed with 12 speed slx to the point I'm not sure id feel any need to spend more money to go to a level above that.