in spite of their claims that someone becoming a JW actually improves marraige, I've read too many accounts of the opposite occuring.
That was very true in our case, too. The WT "improves" a marriage only when both parties make the organization their false god. The couple's first priority is the society, not each other. The couple act not as adult partners, but rather as children under control of a domineering organization. If one partner stubbornly persists in "unbelieving", their mate is to withdraw from them, to "truly hate them", if they are to be true to WT teachings.
What the WT does doesn't improve a marriage. It makes captives of both marital partners if at all possible. If not, they will settle for the half they can get, and the other half becomes "bird seed" to be thrown out on the ground.
Call me bird seed. As the unbelieving party in our marriage, I knew early on that I shouldn't ask her to choose between me and the organization: her first loyalty was to them, not to me. Though I never asked her to make that choice, she finally did. In a way, that's a relief: three is definitely a crowd when the WT is part of the picture.