Strengthening cards is kind of hard and takes a lot of time but your hard work will be paid off once you complete it. You can do plenty of tasks daily, weekly, or rookie tasks especially if you’re a new player to gain things, you can even receive a lot in achievements and fieldwork.
Though, there’s nothing much to do if you already completed the tasks and the main story. Events are mostly the only thing that you’ll do in the game plus the temple of trials, daily & weekly tasks but that will not be the case if you’re already attached to the game. All in all, the game’s pretty good but I hope it will improve more.