Arthur had been planning for many years and was very clear about one thing, seeking revenge on Nasir Town at this point in time was undoubtedly the wisest choice.
"There will never be a better opportunity for revenge in my life!"
Now that a large number of powerful individuals from Cyart were heading to Rhea, the number of enemies within Cyart capable of interfering with and pursuing him was relatively reduced. Moreover, his current level of strength was at the strongest phase that wouldn't be detected by the big barrier.
If he were any stronger, his presence would alarm the entire East Coast the moment he set foot in Cyart, and if he were any weaker, the risks involved in the upcoming battles would drastically increase.
Once he eradicated Nasir Town, Arthur would immediately flee back to Rhea. Even if he were detected re-crossing the big barrier, it wouldn't matter anymore, as no powerful expert from Cyart would have a chance to catch up with him.