Do you actually need to watch every Marvel film?

No. Here is the shortest route through the Infinity Saga that still makes Endgame work.

Twenty-three films is roughly fifty hours. That is a big ask for someone who only wants to understand why everyone was upset in 2019, and the honest answer is that a third of them are optional.

The load-bearing films are the ones that move an Infinity Stone or change who is in the room at the end. Iron Man, The Avengers, Winter Soldier, Guardians of the Galaxy, Civil War, Thor: Ragnarok, Black Panther, Infinity War, Endgame. Nine films, about twenty hours, and nothing in Endgame will be a surprise for the wrong reasons.

The ones we would add back first are Ant-Man, because the mechanism that resolves Endgame is explained there, and Captain Marvel, because she arrives in the last act with no introduction otherwise.

Everything else is enrichment. Very good enrichment in places. But if you are watching to catch up rather than to complete a set, nine is enough.