Proof of concept of UserDefaults plus Zephyr, making use of notifications to update the UI when a value changes externally.

ContentView.swift

import SwiftUI

struct ContentView: View {
  @State private var preferences = PreferencesModel()
  
  var body: some View {
    VStack {
      HStack {
        Text("Value: ")
        Text(preferences.theNumber.formatted()).bold()
      }
      Button(action: changeNumber) {
        Text("Change number")
      }
    }
    .padding()
  }
  
  func changeNumber() {
    preferences.theNumber = Int.random(in: 1...100)
  }
}

#Preview {
  ContentView()
}

PreferencesModel.swift

import SwiftUI
import Zephyr

@Observable
class PreferencesModel {
  
  init() {
    
    /// Watch for changes to UserDefaults.
    /// This will fire when a value changes externally and Zephyr updates
    /// UserDefaults in kind.
    let observer = NotificationCenter.default.addObserver(
      forName: UserDefaults.didChangeNotification,
      object: nil,
      queue: nil,
      using: handleUpdatedUserDefaults
    )
    
    Zephyr.debugEnabled = true
    Zephyr.sync(keys: UserPreferences.Keys.allKeyValues)
    Zephyr.addKeysToBeMonitored(keys: UserPreferences.Keys.allKeyValues)
  }
  
  /// See what changed in UserDefaults and update our model in kind.
  /// This means that when a value changes externally, the UI will update automatically.
  private func handleUpdatedUserDefaults(from notification: Notification) {
    guard let updatedUserDefaults = notification.object as? UserDefaults else {
      return
    }
    let updatedNumber = updatedUserDefaults.integer(forKey: UserPreferences.Keys.theNumber.rawValue)
    if updatedNumber != self.theNumber {
      self.theNumber = updatedNumber
    }
  }
  
  /// Observable var directly linked to UserDefaults.
  /// When it's set from a view (e.g. in ContentView), UserDefaults is automatically updated to match.
  var theNumber = UserPreferences.theNumber {
    didSet { UserPreferences.theNumber = self.theNumber }
  }
}

UserPreferences.swift

struct UserPreferences {
  
  /// Convenience enum for noting all the keys we store in UserDefaults.
  enum Keys: String, CaseIterable {
    case theNumber
    
    static var allKeyValues: [String] {
      Self.allCases.map { $0.rawValue }
    }
  }
  
  /// Standard value stored in UserDefaults.
  static var theNumber: Int {
    get { return UserDefaults.standard.integer(forKey: Keys.theNumber.rawValue) }
    set { UserDefaults.standard.set(newValue, forKey: Keys.theNumber.rawValue) }
  }
}