Venice and Athens are about 700 miles apart. That is the distance from New York to Chicago.
If I am in New York on an international trip, flying to Chicago for a couple of nights seems strange to me. But, sure, it's physically possible.
Of course, Santorini is really far from Athens, too. And, the airport in Athens is really far from the city center. You're setting yourself up for some lengthy travel legs here.
Potentially, it will look like this. On Day 1 you will fly into Athens and be exhausted because it's very far away. It will take you just under an hour to get into Athens center after you land. In a car, that is. Longer if you use public transport.
Athens will be great because you won't have long legs of travel while there.
Getting to Santorini is a chore. You have two choices, ferry or plane. The ferry is eight hours for the big boat or about 5.5 hours for the fast boat. The ferry port is about a 20-minute drive from where you will likely stay in Athens. Flying requires the roughly one-hour haul out to the airport to arrive a few hours early for a quick flight to the island.
Then, what, Venice? Well, good news. There are direct flights from Santorini to Venice, just not every day and with a limited amount of seats. I would get on that.
This is far from unrealistic but that's what you're looking at. Myself? I land in Athens, hop in a car, tour the mainland (thousands of possible places to see vs. Santorini's dozens), drive into Athens, and then drive myself to the airport, drop car, leave.