Is it permissible for someone who delayed making up for missed days of Ramadan due to ignorance of the necessity of expiation to suffice with only repentance and making up the days?

As long as you were unaware of the impermissibility of delaying the make-up fast, you are only required to make up the days you owe, and the expiation for delay is not binding upon you. However, if the expiation were binding, it would be feeding one poor person one mudd (a measure) of food for each day.