Hi, Mrs. Mamby, and welcome!
I’m sorry youo’re having crumbly cakes! The only things I can think of, since you’re using a Rose recipe, is any of the following are possible:
1] Are you measuring, instead of weighing? If so, you could possibly have more dry ingredients or less moist ingredients.
2] When you mix your cocoa with boiling water, are you being sure to cover it tightly until it comes to room temp? Believe it or not, enough liquid can evaporate to dry the cake.
3] Possibly overbaking?
4] Are you using cake flour or bleached all purpose?
I can’t think of anything else. I’ve made the chocolate butter, but not the wedding cake version and not syruped, and I didn’t have any crumbly problems with it.
I hope maybe some of that is helpful. Hopefully, someone else will have more ideas.