If you needed to see another island I would see Naxos as Mykonos shall be very quiet in April and there’s extra to see in Naxos. Most ferries from Santorini to Athens cease in Naxos so it’s simple to make a 1 or 2 night time stopover. I often advocate a day to or two extra in Santorini but since seashores are a precedence then 4/4 is ideal. So we are trying to fly into Milan for a quantity of days, and then Milan to Santorini for 4 days, then Ferry over to Mykonos for 4 days.
Southern Spain is slightly warmer than Greece however not by a lot. You won’t be finding any true hot summer time days anywhere in Europe that early within the season. I would go away the sightseeing in Athens until the top (this also gives you a safety day should any ferries be canceled so that you won’t miss your flight home). I would keep the evening on the Sofitel at Athens Airport and catch the primary flight in the morning to Santorini.