I don't know where or when the force should be applied. But IMO, you need to be hitting a ball off a tee. (long tee when possible) Can't help but think you are limiting your swing potential hitting into the punching bag thing. I think you have to release/throw/whip the barrel. You have to subconsciously firm up your hand grip just prior to hitting the punching bag to avoid injury. Squeezing the bat prior to impact impedes the whipping/throwing of the barrel. Think about those gifs where the hitters right hand is off the handle. Also not being able to feel / see /experience your follow thru has to hurt your development. I think a punching bag can be used to check/ understand/ teach a good contact position but would limit it use to that.