Is expiation (kaffarah) – in addition to making up the missed days (qada') – obligatory for someone who delayed making up days they missed in Ramadan due to a legitimate Sharia-recognized excuse until another Ramadan began, because they were unaware of the obligation to make up the days?

You are not obligated to pay the fidyah (expiatory feeding) due to your ignorance of the impermissibility of delaying making up missed fasts until the next Ramadan arrives. However, you must make up the fasts you missed. Similarly, your not fasting in the second half of Sha'ban, due to your ignorance that it is permissible to make up fasts during that time, does not obligate you to pay the fidyah.