If these 2 routes which i suggested is not feasible than better option will be 3rd Vande Bharat between Chennai - Coimbatore - Ernakulam - Allapuza. This route is tourist attraction due to Back water House Boat in Kerala. It will be a good route.
Chennai - Bangaluru - Mysore route has already Shatabdi that's why not opted that route. Rather it can be replaced by Tejas or future vande bharat.