The problem with that is he's stagnating just staying in the capital.
All of these adventures are powering him up so he'll be ready to face the trials that are coming. Without Eliza getting crippled and his voyage to get the chalice, he would have missed out on so much growth, such as the healing crystals. Same with going to rescue Kira/Pearl. Diana is exactly the kind of sparring partner he needs because he's outgrown Eliza martially. All these baubles and magic he's picking up on the island will help him in his eventual fight with Taron and whatever lies beyond. We, as the player, know that Taron is a bit player in a grand scheme, and while Taron is definitely written weak, there's no telling what the next bigger threat will be like and the MC should be as prepared as possible for it.