Dave, 5 lessons is also my favorite book, and I understand your breaking of the wrist.
I had that when I read the book, and that's because I supinated the left wrist
forcefully.
I found out that when I downswing and
let the club release, it is much easier to supinate the wrist, instead of me forcing the release by supinating the wrist (lol, I hope I make sense

).
Remember, your hands need to accelerate through impact. Don't think about your hands position at impact as it will cause your hands to slow down. Continue supinating well after impact.
Actually, there's not much wrist/forearm muscle involved to supinate the wrist (you should see my tiny forearms).