Do you need to watch every Marvel movie before Avengers: Doomsday?

No — you do not need to watch every Marvel movie before Avengers: Doomsday.

A better approach is to focus on the big setup:

• Loki seasons 1–2: the multiverse, timelines, TVA
• Spider-Man: No Way Home: multiverse crossovers in an easy-to-follow way
• Doctor Strange in the Multiverse of Madness: introduces incursions
• Deadpool & Wolverine: more multiverse rules and TVA context
• Infinity War and Endgame: for the Tony Stark/Robert Downey Jr. legacy angle
• The Fantastic Four: First Steps: useful for Doom and the Fantastic Four setup

Bottom line: if you know the multiverse basics and the big Avengers payoff, you should be fine. For the fullest context, check the current release order and watch the key movies above, but you do not need a full MCU marathon.