This year I lifted heavy until July. Which is way longer and my hr were down because my swing was not consistent.
Took July off from lifting and hr's came back+
I think if you spent a lot of time stretching you could lift heavy all year. If you can stay loose do it.
As for specific lifts for hitting, I just get as strong all over and use bp and other techniques such as hitting basketballs and swinging a bat with fins.
go to my videos in youtube and search for my video called distance training.