I have had good luck with large plants like that doing what I call spade pruning . take a sharp spade and just cut straight down the center and lift half the plant out . replace ''good ''soil in the hole.
Then you can take a sharp knife and divide the 1/2 you lifted out into smaller pieces. I have done this in the spring and fall, but sounds like you don't have enough rain this year to try a fall transplant.
Good luck and keep us posted
