Hiding the keys is a start, also look up ways your mum can do all her stuff without having to use a car , how to use internet shopping, organise a system of people giving her rides, look up taxis for the elderly, and other public transport. Otherwise she will feel stuck and will rightfully complain.
Your mums right to her independence is not greater than someone’s right to not be hurt or killed by her driving when she shouldn’t be. We live in a society where we gotta look after each other. If she doesn’t understand that, or won’t listen to you explain it the.
You have 110% done the right thing.