Plane or ferry? It depends on many parametres. In June/July of a couple of years ago (at the end of the longest Covid lockdown in Greece), my wife, our two dogs and I decided to use our car and a lot of ferries to "combine" a lot of Ionian (8 of them) islands on a single trip. Everything was very smooth, even the Corfou-Erikousa and back legs, that I booked at the harbour agency in Corfou. Both of us being retired, time did not interfere with our planless trip that lasted a bit longer than 5 weeks. Had we have 2 weeks available (like in your case) we simply could not have done it. For example, to get from Lefkada to Corfou the direct ferry is difficult to use, as it only goes twice (or three times) weekly. So you have to cross by bus to Prevesa, and take another bus to Igoumenitsa to get over to Corfou. If you want to also visit Paxoi and one (or more) of the Diapontian islands you have to do that using Corfou as a base. On the other hand, if you're in Crete (like we are or you might be), you can take the 6:30 am flight to ATH, connect at CFU around 9 and be there at 10 am. In other words, you need some research, especially since you seem to want to "do" a lot of islands in the Docecannese, Ionian, Cyclades and Crete, all of those in a couple of weeks. Still doable, but believe me, after returning home you'll need a vacation!

As for Google Flights in April, I can't answer that, I can only say that although A3 usually releases next year's schedule in late summer, they don't really change much in terms of domestic travel, so you can always do your own preliminary scheduling of your vacation. Keep in mind that ferries have a much reduced schedule in April compared to the summer...