What might a cross-platform GUI app look like with Swift? I'm assuming SwiftUI can't be used? Regardless, I'll definitely be reaching for Swift for one-off scripts. I've enjoyed the times I needed to use Swift for a React Native extension.
After exploring most of these for my use case (something that needs to run on an "embedded" RPi CM4 as well as iOS), I wrote a Swift bridge/runner for Flutter [7] which is working well. That enables me to write the UI in Dart and the business logic in Swift. I looked at all the other options below (that's how I know about them, indeed [3] I wrote as part of my research), in the end I felt that none of them were at the time of investigating sufficiently mature to base a product around.
[1] https://github.com/rhx/SwiftGtk
[2] https://github.com/Longhanks/qlift
[3] https://github.com/PADL/LVGLSwift
[4] https://github.com/TokamakUI/Tokamak
[5] https://github.com/stackotter/swift-cross-ui