Santorini in first week of April or not

With its 15,000 permanent inhabitants there is activity in Santorini all year round.

First week of April, around 15,000 cruise passengers are expected, as many as inhabitants. Plus other visitors by ferry or plane.
Same answer regarding Mykonos with a little fewer visitors.

I don't know what "too cold" means to you. Expect between 55F and 65F on average during the day

Thank you for the response. I am also looking for option to stay either in oai or Fira. During this time of the year should I have a preference of 1 over the other in terms of restaurants open and businesses being open as when I am reading blogs it says businesses and restaurants are closed for winter and they just start to open

I don't know which blog you are looking at, but given that you are not going to book a hotel via these blogs you should instead consult platforms like booking.com and specify your dates. Open the map and tick the box "only show available properties".

You will see a large number of accommodations open in the first week of April.
Book directly with the hotel via its website if available

The places I stayed in Santorini being neither in Oia nor in Fira, I cannot recommend one or the other.
Fira, the capital, is livelier than Oia with more shops and restaurants open. Additionally it's the hub for buses.

Thank you for your response. We have 2 days in Mykonos and 2 days in Santorini. Would love to hear recommendations on what to do and where to stay in these 2 places.

You probably should be looking at Fira it probably will have more facilities open that early in the season.
Which island are you planning to go to first? There is only one ferry scheduled to do that run with the earliest departure in the morning from Mykonos. The ferry from Santorini departs in the afternoon, Either way you will use up half a day. You might have a better trip if you just stayed in one Island.

What to do. In Santorini you might want to go to the ancient Minoen site at Acroteri. If you stay in Fira you can take the bus to Oia and explore the village.

In Mykonos you might be able to take the excursion ferry to the ancient site at nearby Delos.

Athens to Mykonos morning ferry and then from Mykonos to Santorini morning ferry that reaches around early afternoon. Flying late night from Santorini to Athens so we could use the whole 3 rd day pretty much.

2 days in Mykonos n and 2 days in Santorini Is doable right? Mykonos is pretty small island n should be able to cover it by foot. For Santorini can stay in Fira n explore Fira N oai by bus, taxi n all. Go-ferry.com shows atleast one morning n one evening decent fast ferry options. Hope that’s doable

Mykonos is pretty small island in should be able to cover it by foot

Of course, a quick tour of the island will only take about 10 hours non stop on foot with only 1500 meters of cumulative elevation gain.

You can shorten the walk by only going from the port to the eastern part, it will take only 8 hours round trip.