Ride your bike !
Chuck it about, shift your weight around practice hopping logs manualing and front wheel wheelies and enjoy it.
This will improve your stability whilst you are actually riding, it doesent involve kettle bells and most importantly you don’t have to go down the Gym where strange folk lurk around touching themselves whilst looking in the mirror.
I don’t want to upset any one but your wasting your time, the earth has a core and so do apples, I was told this years ago, seeing as we don’t have one then what your actually doing is fuvking your back up.
If you have spare time then spend it on two wheels ?