Orthodox Easter falls on 2 May 2021. My intended month on Crete will probably need to include that week, so I request advice on how to adjust to the expected large increase of domestic travelers. When usually does Greek Easter travel begin and when end? I wish to roam the island by bus and walking, prefer to stay in simple lodging and have zero desire for "night life". I might have opted to stay only during April, but Samaria Gorge is closed until 1 May.

The islands don’t get busy until foreign visitors begin arriving for their summer holidays. Many hotels and restaurants don’t even open for Easter, but probably more than usual in 2021 because it falls in May. The only thing you have to worry about is booking ferries and flights well in advance around the Easter week, especially for travel to the islands before Easter and back to Athens afterwards.

Greeks like to return to the village or island where their family lives for Easter, usually staying in the family home. Tavernas opening for Easter might be busy, and Greeks like to linger over their meals. Easter on the islands isn’t nearly as busy as summer high season.

Greeks travel to specific islands such as Mykonos ,Santorini, Spetses, Hydra etc for Easter, but Crete is not one of them.
Apart from that Crete is a very big island and finding accommodation in May is not going to be a problem even during Easter.
The big wave of traveling begins on Holy Thursday returning back to the big cities 1- 2 days after Easter Sunday.
Crete is the destination with the longest tourist season in Greece, from March to early November, May is still a relatively quiet month however.
regarding the Samaria Gorge, even though it opens on May 1st ( which in Greece and in generally in Europe is the Labour Day and a bank holiday), this also depends on the accessibility in the gorge...
If the water level of the Samaria river is too high, the Gorge will remain closed.