If you lock server transfers now, it’s going to be super imbalanced. The damage is already done.
My wild suggestion (but just might work) is to randomly sort guilds into all servers evenly when free transfers end. I know it would cause some unrest but it would die down and would permanently fix the mess you’ve made with free transfers all this time. It would also make everyone have a reasonable (or no) wait time and fix night capping imbalances.
Give’er a go!