But the thing about Zwift's trainer difficulty slider is that it affects both uphill and downhill. At 50%, a 10% climb on Zwift would feel like a 5% climb and a -10% descent will only feel like a -2.5% descent. Which may actually be a good thing when racing... but just throwing it out that the cassette swap analogy is only half the story.
Zwift Trainer Difficulty on Descents: Did You Know? - Zwift Insider