I just transferred to Balmung(!), so Legacy worlds apparently aren't full yet. (Yay!)

If there's a "trick", other than extreme luck, it's that you want to refresh your browser until you get your characters listed on the start page of MogStation, before you click Optional Services / World Transfer. It wasn't working earlier, so maybe I just got extremely lucky. Or maybe right now is just less crowded between Europe and North America prime times.

Good Luck!