Assalamualaykum Ustadh, if someone missed fasts last Ramadan due to a stomach illness and is currently unable to make up those missed fasts before the next Ramadan because they are still recovering, what should they do in this situation?

If recovery is expected, the fasts stay a debt on you until you are able, even if that is after years, and since the delay is due to illness there is no expiation for it. If doctors confirm the illness will not go away, you feed one poor person for each day and do not make them up.