SwiftUI Alert With Countdown - How to Dynamically Update Variable Used in Alert

In my SwiftUI view below, I would like my alert to update with the countdown variable which controls whether or not the Proceed button is active & also shows up in the text of the alert itself.

The current implementation does not update the value of countdown - it always remains as 5.

struct RedemptionView: View {
    @Environment(\.dismiss) var dismiss
    @State private var showQRCodeScanner = false
    @State private var showPasscodeView = false
    @State private var billAmount: String = ""
    @State private var isShowingRedemptionStepView = false
    @State private var showAlert = false
    @State fileprivate var countdown: Int = 5

    
    var allowsDelitePointCollection: Bool
    var voucherTitle: String
    var discountId: Int
    
    var body: some View {
        VStack {
            DismissButton()

            VStack(spacing: 16) {
                Text("Scan the QR code provided by the staff, or pass your phone to them to enter the staff code.")
                    .font(.subheadline)
                    .multilineTextAlignment(.center)
                    .padding(.horizontal)
                    .fixedSize(horizontal: false, vertical: true)
                
                Button(action: {
                    if !billAmount.isEmpty {
                        showAlert = true
                        startCountdown()
                    } else {
                        showQRCodeScanner = true
                    }
                }) {
                    Text("Scan QR Code")
                        .font(.headline)
                        .padding()
                        .frame(maxWidth: .infinity)
                        .background(Color.blue)
                        .foregroundColor(.white)
                        .cornerRadius(10)
                }
                .padding(.horizontal)
                
            }
            .padding(.bottom)
            
            Spacer()
        }
        .onTapGesture {
            hideKeyboard()
        }
        .padding(.top, 20)
        .sheet(isPresented: $showQRCodeScanner) {
            QRCodeScannerView(discountId: discountId, billAmount: billAmount, discountTitle: voucherTitle, isPresented: $isShowingRedemptionStepView)
        }
        .sheet(isPresented: $showPasscodeView) {
            PasscodeView(billAmount: billAmount, discountId: discountId, discountTitle: voucherTitle, isPresented: $isShowingRedemptionStepView)
        }
        .onChange(of: isShowingRedemptionStepView) { oldValue, newValue in
            if !newValue && oldValue {
                dismiss()
            }
        }
        .alert("Verify Bill Amount - \(priceToStrFormat(price: (Int(billAmount) ?? 0) * 100, useComma: false))", isPresented: $showAlert) {
            Button("Proceed\(countdown > 0 ? " In \(countdown)s" : "")") {
                if countdown == 0 {
                    showQRCodeScanner = true
                }
            }.disabled(countdown > 0)
            Button("Cancel", role: .cancel) { }
            } message: {
                Text("Show Phone to Staff to verify bill amount. Proceed in \(countdown) seconds")
            }
    }


    private func startCountdown() {
        Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { timer in
            if countdown > 0 {
                countdown -= 1
            } else {
                timer.invalidate()
            }
        }
    }
}