It is quite common for people treated (successfully) for AF by catheter ablation to develop flutter. I know, it seems disappointing and bizarre, but the electrophysiologists know this and can usually correct it with a second ablation, but a different kind or process and area. The results are usually pretty good.
Atrial enlargement can be a RESULT of atrial fibrillation and it can also CAUSE atrial fibrillation. They go hand-in-hand quite often. It is true that advanced AF, or its subsequent effects, can make a successful ablation more difficult, but the skill and experience of the EP are what matters most in these cases. Some EPs simply don't have the experience or the confidence to perform the more advanced and more difficult ablations. The very best won't have too much problem. You'll have to do some research to find EPs nationally who specialize in difficult cases ('difficult' having a range of implications because it's a relative descriptor, not a precise one). If you have resources and don't mind some traveling and commercial accommodations for up to a week, I can recommend two EPs with national and international recognition who are in the USA, and two who are in France, one of whom trained one of the two in the USA.