I solved my problem by freeing up more space on my Mac by uninstalling large applications and reinstalling them after the Xcode update was successful. In the end, before updating Xcode through the App Store I had about 4x - 5x more disk space than Xcode actually needed. For anyone else I would still suggest to follow @jva´s answer, because I think that my solution was very device specific. But if nothing works I would suggest to try out freeing more space on your device.

I seem to have found the solution myself. Setting .preferredColorScheme(.light) on a SwiftUI View disables dark mode.