Why short, timed drills work
Knowing a rule and using it quickly are different skills. In a conversation you don’t have time to think through the rule for ser or estar; you need the right form to come out automatically. Short timed rounds build that speed. Because every mistake shows the rule straight away, you correct the misunderstanding while it’s fresh, instead of practising the error.
A good routine is one or two rounds a day on the drill you find hardest. Read your miss list after each round, then play again. When your score stops climbing, switch from tap mode to typing mode, which also trains spelling and accents.